




















































naval r.esear,ch 

LABOR.ATOR.Y 








SUMMARY TECHNICAL REPORT 
OF THE 

NATIONAL DEFENSE RESEARCH COMMITTEE 


LC REGULATION: BEFORE SERVICING 
OR REPRODUCING ANY PART OF TliHj 
DOCUMENT, ALL CLASSIFICATION 
MARKINGS MUST 6E CANCELLEX 


This document contains information affecting the national defense of 
the United States within the meaning of the Espionage Act, 50 U. S. C., 
31 and 32, as amended. Its transmission or the revelation of its contents 
in any manner to an unauthorized person is prohibited by law. 

This volume is classifiedH^^®lf in accordance with security regula¬ 
tions of the War and Navy Departm ents because certain chapters 
contain material which was^jM^^V the date of printing. Other 
chapters may have had a lower classification or none. The reader is 
advised to consult the War and Navy agencies listed on the reverse of 
this page for the current classification of any material. 






Manuscript and illustrations for this volume were prepared 
for publication by the Summary Reports Group of the 
Columbia University Division of War Research under con¬ 
tract OEMsr-1131 with the Office of Scientific Research and 
Development. This volume was printed and bound by the 
Columbia University Press. 

Distribution of the Summary Technical Report of NDRC has 
been made by the War and Navy Departments. Inquiries 
concerning the availability and distribution of the Summary 
Technical Report volumes and microfilmed and other refer¬ 
ence material should be addressed to the War Department 
Library, Room lA-522, The Pentagon, Washington 25, D. C., 
or to the Office of Naval Research, Navy Department, Atten¬ 
tion : Reports and Documents Section, Washington 25, D. C. 

Copy No. 

177 


This volume, like the seventy others of the Summary Tech¬ 
nical Report of NDRC, has been written, edited, and printed 
under great pressure. Inevitably there are errors which have 
slipped past Division readers and proofreaders. There may 
be errors of fact not known at time of printing. The author 
has not been able to follow through his writing to the final 
page proof. 

Please report errors to: 

JOINT RESEARCH AND DEVELOPMENT BOARD 
PROGRAMS DIVISION (STR ERRATA) 

WASHINGTON 25, D. C. 

A master errata sheet will be compiled from these reports 
and sent to recipients of the volume. Your help will make this 
book more useful to other readers and will be of great value 
in preparing any revisions. 


SUMMARY TECHNICAL REPORT OF DIVISION 13, NDRC 


VOLUME 3 


SPEECH AND FACSIMILE 
SCRAMBLING AND DEC|^ING 

By, authority Secretary of 


SEP 2 31960 

memo 2 August I960 


OFFICE OF SCIENTIFIC RESEARCH AND DEVELOPMENT 
VANNEVAR BUSH, DIRECTOR 


NATIONAL DEFENSE RESEARCH COMMITTEE 
JAMES B. CONANT, CHAIRMAN 

DIVISION 13 
HARADEN PRATT,CHIEF 


^sesg 


WASHINGTON, D. C., 1946 



LC REGULATION: BEFORE SERVICING 
OR REPRODUCING ANY PART OP THIS 
DOCUMENT, ALL CLASSIFICATION 
MARKINGS MUST~BE CANCELLEgf 






NATIONAL DEFENSE RESEARCH COMMITTEE 


James B. Conant, Chairman 
Richard C. Tolman, Vice Chairman 
Roger Adams Army Representative^ 

Frank B. Jewett Navy Representative- 

Karl T. Compton Commissioner of Patents^ 

Irvin Stewart, Executive Secretary 


'^Army representatives in order 


Maj. Gen. G. V. Strong 
Maj. Gen. R. C. Moore 
Maj. Gen. C. C. Williams 
Brig. Gen. W. A. Wood, Jr. 


of service: 

Col. L. A. Denson 
Col. P. R. Faymonville 
Brig. Gen. E. A. Regnier 
Col. M. M. Irvine 
Routheau 


Col. E. A. 


^Navy representatives in order of service: 

Rear Adm. H. G. Bowen Rear Adm. J. A. Furer 
Capt. Lybrand P. Smith Rear Adm. A. H. Van Keuren 
Commodore H. A. Schade 
^Commissioners of Patents in order of service: 
Conway P. Coe Casper W. Corns 


NOTES ON THE ORGANIZATION OF NDRC 


The duties of the National Defense Research Committee 
were (1) to recommend to the Director of OSRD suit¬ 
able projects and research programs on the instrumen¬ 
talities of warfare, together with contract facilities for 
carrying out these projects and programs, and (2) to 
administer the technical and scientific work of the con¬ 
tracts. More specifically, NDRC functioned by initiating 
research projects on requests from the Army or the 
Navy, or on requests from an allied government trans¬ 
mitted through the Liaison Office of OSRD, or on its own 
considered initiative as a result of the experience of its 
members^ Proposals''p;repared by the Division, Panel, or 
Committee for research contracts for performance of 
the work involved ip such projects were first reviewed 
by NDRC, and if approved, recommended to the Director 
of OSRD. Upon approval of a proposal by the Director, 
a contract'permittirig’ maximum flexibility of scientific 
effort was arranged. The business aspects of the con¬ 
tract, including such matters as materials, clearances, 
vouchers, patents, priorities, legal matters, and admin¬ 
istration of patent matters were handled by the Execu¬ 
tive Secretary of OSRD. 

Originally NDRC administered its work through five 
divisions, each headed by one of the NDRC members. 
These were: 

Division A—Armor and Ordnance 
Division B—Bombs, Fuels, Gases, & Chemical Problems 
Division C—Communications and Transportation 
Division D—Detection, Controls, and Instruments 
Division E—Patents and Inventions 


In a reorganization in the fall of 1942, twenty-three 
administrative divisions, panels, or committees were 
created, each with a chief selected on the basis of his 
outstanding work in the particular field. The NDRC 
members then became a reviewing and advisory group 
to the Director of OSRD. The final organization was as 
follows: 

Division 1—Ballistic Research 

Division 2—Effects of Impact and Explosion 

Division 3—Rocket Ordnance 

Division 4—Ordnance Accessories 

Division 5—New Missiles 

Division 6—Sub-Surface Warfare 

Division 7—Fire Control 

Division 8—Explosives 

Division 9—Chemistry 

Division 10—Absorbents and Aerosols 

Division 11—Chemical Engineering 

Division 12—Transportation 

Division 13—Electrical Communication 

Division 14—Radar 

Division 15—Radio Coordination 

Division 16—Optics and Camouflage 

Division 17—Physics 

Division 18—War Metallurgy 

Division 19—Miscellaneous 

Applied Mathematics Panel 

Applied Psychology Panel 

Committee on Propagation 

Tropical Deterioration Administrative Committee 


iv 



2015 


490943 






NDRC FOREWORD 


A s EVENTS of the years preceding 1940 re- 
. vealed more and more clearly the serious¬ 
ness of the world situation, many scientists in 
this country came to realize the need of organ¬ 
izing scientific research for service in a na¬ 
tional emergency. Recommendations which they 
made to the White House were given careful 
and sympathetic attention, and as a result the 
National Defense Research Committee [NDRC] 
was formed by Executive Order of the Presi¬ 
dent in the summer of 1940. The members of 
NDRC, appointed by the President, were in¬ 
structed to supplement the work of the Army 
and the Navy in the development of the instru¬ 
mentalities of war. A year later, upon the 
establishment of the Office of Scientific Re¬ 
search and Development [OSRD], NDRC be¬ 
came one of its units. 

The Summary Technical Report of NDRC is 
a conscientious effort on the part of NDRC to 
summarize and evaluate its work and to present 
it in a useful and permanent form. It comprises 
some seventy volumes broken into groups cor¬ 
responding to the NDRC Divisions, Panels, and 
Committees. 

The Summary Technical Report of each Di¬ 
vision, Panel, or Committee is an integral sur¬ 
vey of the work of that group. The first volume 
of each group’s report contains a summary of 
the report, stating the problems presented and 
the philosophy of attacking them, and summar¬ 
izing the results of the research, development, 
and training activities undertaken. Some vol¬ 
umes may be ‘^state of the art” treatises cover¬ 
ing subjects to which various research groups 
have contributed information. Others may con¬ 
tain descriptions of devices developed in the 
laboratories. A master index of all these di¬ 
visional, panel, and committee reports which 
together constitute the Summary Technical Re¬ 
port of NDRC is contained in a separate vol¬ 
ume, which also includes the index of a 
microfilm record of pertinent technical labora¬ 
tory reports and reference material. 

Some of the NDRC-sponsored researches 
which had been declassified by the end of 1945 
were of sufficient popular interest that it was 
found desirable to report them in the form of 
monographs, such as the series on radar by Di¬ 
vision 14 and the monograph on sampling 
inspection by the Applied Mathematics Panel. 
Since the material treated in them is not dupli¬ 
cated in the Summary Technical Report of 


NDRC, the monographs are an important part 
of the story of these aspects of NDRC research. 

In contrast to the information on radar, 
which is of widespread interest and much of 
which is released to the public, the research on 
subsurface warfare is largely classified and is 
of general interest to a more restricted group. 
As a consequence, the report of Division 6 is 
found almost entirely in its Summary Technical 
Report, which runs to over twenty volumes. The 
extent of the work of a Division cannot there¬ 
fore be judged solely by the number of volumes 
devoted to it in the Summary Technical Report 
of NDRC; account must be taken of the mono¬ 
graphs and available reports published else¬ 
where. 

Of all the NDRC Divisions, few were larger 
or charged with more diverse responsibilities 
than Division 13. Under the urgent pressure of 
wartime requirements, the staff of the Division 
developed navigation and communications de¬ 
vices and systems which not only contributed to 
the successful Allied war effort, but which will 
continue to be of value in time of peace in the 
fields of transportation and communications. 
The work of the Division, under the direction 
first of C. B. Jolliffe and later of Haraden Pratt, 
furnishes a foundation for what promises to be 
even more radical developments than those of 
the war—for one example, direction finders 
which will operate at all elevations and azimuth 
angles, in other words, hemispherically. 

The Summary Technical Report of Division 
13 was prepared under the direction of the Di¬ 
vision Chief and authorized by him for publica¬ 
tion. The report presents the methods and 
results of the widely varied research and de¬ 
velopment program, and, in the case of work 
with speech scrambling and decoding, it pre¬ 
sents for the first time a comprehensive review 
of the state of the art. The report is also a 
notable record of the skill and integrity of the 
scientists and engineers, who, with the coopera¬ 
tion of the Army and Navy and Division con¬ 
tractors, contributed brilliantly to the defense 
of the nation. To all of these we express our 
sincere appreciation. 

Vannevar Bush, Director 
Office of Scientific Research and Development 

J. B. CONANT, Chairman 
National Defense Research Committee 


V 




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FOREWORD 


E arly in October 1940 a subcommittee on 
speech secrecy was set up in the Communi¬ 
cations Division of the National Defense Re¬ 
search Committee [NDRC], later known as 
Division 13 of NDRC. This group was to con¬ 
sider both the scrambling and unscrambling of 
telephone signals. It was soon recognized that 
the decoding problem was of primary impor¬ 
tance both as a means of evaluating privacy 
systems for possible use by the Services and for 
decoding possible enemy signals. Thus the work 
under Division 13 on speech privacy took on two 
aspects, that of providing “secure” means of 
voice, code, and picture communication, and 
that of decoding transmissions for the dual 
purpose of learning how useful were our own 
secrecy systems and for learning what the 
enemy might be saying in his scrambled mes¬ 
sages. 

Numerous projects were carried forward 
under the guidance of the Division, some of 
them with the object of developing scrambling 
or secrecy methods, others dealing entirely with 
code cracking methods. In this way the work 
accomplished on methods of secrecy could be 
tested at every phase by the decoding groups 
which were also busy on other aspects of the 
decoding problem. The summaries to follow will 
disclose the objects and accomplishments of the 
individual projects. 

Work on speech secrecy problems went on in 
the Division from 1940 quite to the end of the 
war, the final report on Project 13-106 being 
issued on August 18, 1945. Thus, this volume 
summarizes the results of about five years' ex¬ 
perience in developing secrecy systems and in 
diagnosing, decoding, and evaluating these and 
other systems submitted for study by the Army, 
the Navy, and NDRC. 

All these studies are reported in detail in the 
preliminary and final reports on the projects 
and are only summarized here. The sum of 
these reports, therefore, provides a record of 
accumulated experience, much of which has 


never been reported in any other way. In toto, 
the reports make available information both 
positive and negative which would have to be 
accumulated by another group if they were to 
embark on a similar project. 

The immediate pressure behind these studies 
was caused, of course, by the war. The work 
summarized in this volume, therefore, should 
serve as a guide to the individual reports and 
should aid a newcomer to the field in becoming 
properly oriented to the state of the art so far 
as the NDRC project reports are concerned. 
Other work, carried out in other government 
groups, is not believed to duplicate this material. 

In passing, it is worth noting that, in con¬ 
trast to a rather extensive literature on code 
and cipher systems, and on cryptanalysis and 
cryptography, which apply to telegraphic types 
of communication, very little has been written 
on speech privacy systems or decoding methods 
applying to them. Two moderately comprehen¬ 
sive articles have been published. One appeared 
in the Post Office Engineers Journal, October 
1933; the other, in the Brown Boveri Review 
for December 1941. The latter report is repro¬ 
duced and discussed in Preliminary Report No. 
5, Project C-43. It covers a number of basic 
types of scrambling systems and, in addition, 
discloses one that was new at the time. This is a 
modification of the “time division scrambling” 
or TDS system, and several of the Division 13 
projects were concerned with this useful 
privacy method. 

Without any doubt, one of the major accom¬ 
plishments of the Division’s work and certainly 
the high point in its efforts within the subject 
matter of this volume was the development of 
the sound spectrograph by Mr. R. K. Potter and 
his co-workers at the Bell Telephone Labora¬ 
tories. The efficacy of this instrument in ana¬ 
lyzing scrambled speech will be revealed in the 
chapters to follow. 

Haraden Pratt 
Chief, Division 13 



vii 





PREFACE 


I N SUMMARIZING the several hundred reports 
of contractors on the hundred-odd research 
projects sponsored by Division 13 of the Na¬ 
tional Defense Research Committee, the editor 
has had to settle in his own mind how much or 
how little of each project report should be in¬ 
cluded; in other words, how far the boiling- 
down process should go. 

The editor has an abhorrence for seeing good 
scientific or technical material go unpublished. 
Only by publication can the facts or methods 
developed by a few researchers become avail¬ 
able for all researchers. On this basis, substan¬ 
tially all of Division 13’s program should be 
included in the volumes, of which this is one, 
summarizing the work of the Division. On the 
other hand, time moves forward inexorably so 
that it is quite likely that, by the day of publica¬ 
tion, much of the data would already be out 
of date. Furthermore, time and human energy 
are always scarce. On these bases, all that might 
be required would be a paragraph or two sum¬ 
marizing the aims of the project and its accom¬ 
plishments. 

A middle course was steered, a course between 
the easiest solution of publishing practically all 
of each report and the more difficult job of 
really digesting the project purpose and results. 
The editor, however, deliberately chose to pub¬ 
lish too much rather than too little. In most 
cases it will be unnecessary for the reader to 


search out the original source material unless 
he wishes to dig deep into the subject. In those 
cases where fundamental information was as¬ 
sembled and printed in the project report, that 
is, information on which future research might 
be based, the summaries have been permitted to 
take as much space as required. 

The plan followed in this volume is briefly 
as follows: After a comprehensive descrip¬ 
tion of the several scrambling methods, brief 
summaries are given of the work carried out in 
the several projects dealing with scrambling. 
Then follows a comprehensive description of 
decoding or cracking methods employed, after 
which are summaries of the projects dealing 
mainly with decoding or code cracking. In other 
words, most of the space is devoted to basic ma¬ 
terial and the least space to details of the actual 
work carried out. Thus this volume might be 
considered as a basic text on speech scrambling 
and descrambling. 

The material is not arranged chronologically 
with respect to the order of the work in the Di¬ 
vision but, rather, in an arrangement which 
appeared to the editor as giving the reader the 
easiest and quickest approach to the whole sub¬ 
ject of speech privacy and code cracking. Thus, 
parts of the individual project reports will be 
found in several places in this volume. 

Keith Henney 
Editor 


\ 


IX 






CONTENTS 


CHAPTER page 

1 Speech Scrambling Methods. 1 

2 Time Division Scrambling Systems . . . . 13 

3 Speech Privacy System Development .... 25 

4 Unscrambling and Decoding Methods ... 35 

5 Decoding Projects.100 

6 Facsimile Privacy Systems.105 

7 Miscellaneous Projects.120 

Bibliography.125 

OSRD Appointees.127 

Contract Numbers.128 

Service Project Numbers.129 

Index.131 














Chapter 1 

SPEECH SCRAMBLING METHODS 


A WIDE VARIETY of speech scrambling methods 
will be examined in this chapter, taken 
from Part I of the final report^ of Project 
C-43, in order to become familiar with the 
devices which might be used alone or in combi¬ 
nation to make up speech privacy systems. 
Some of these systems are in commercial or 
military use, others exist only on paper, mostly 
in the form of patents or patent applications. 
It is not intended to include all the variations 
of all the different methods but rather to cover 
basic scrambling methods, with their most im¬ 
portant variations, in which the original speech 
is transmitted with its parts modified, dis¬ 
placed, or interchanged. 

The two main dimensions of speech which are 
operated upon to make it unintelligible are the 
frequency dimension and the time dimension. 
Scrambling systems usually depend on re¬ 
arranging the components of speech in either 
or both of these dimensions. In general it may 
be said that those that operate on the frequency 
dimension alone are capable of the best quality 
in the reproduced speech. A complete list of 
the systems covered in the discussion is given 
in Table 1 of Chapter 4, together with other 
data concerning them. 


SYSTEMS INVOLVING SINGLE 
MODULATION 

A basic device in privacy systems is the 
modulator. One form of modulator, shown in 


the coils as shown. In some cases the coils can 
be omitted as shown in Figure IB. 

Figure 2 shows the method of producing 
simple inversion. In this and in succeeding 
illustrations the numerical values are not neces¬ 
sarily the best values for practical operation, 
but they serve to illustrate the manner in which 
the device operates. 

In the system shown in Figure 2 the speech 
band is limited to 3 kc by a low-pass filter. It 
is then modulated with a frequency of 3 kc. 
This produces an upper and a lower sideband of 
which only the latter is passed by the output 
filter. The system is called inversion, because 
the high frequencies in the original speech 
appear as low frequencies in the output and 
the low frequencies in the original speech 
appear as high frequencies. At the receiving 
end the inverted signal, in passing through an 
identical system in the same direction, is re¬ 
inverted back to normal speech. 

A very commonly proposed variation of this 
system involves using a variable frequency in¬ 
stead of the steady 3-kc carrier. We might vary 
the frequency continuously or in discrete steps. 
It should be noted, however, that the cutoff of 
the low-pass output filter is fixed, which limits 
the variation permissible in the carrier fre¬ 
quency. A wide variation would either permit 
too much of the upper sideband to get through 
or would cut off some of the lower sideband. 

If the modulator in Figure 2 is of the type 
shown in Figure lA, speech can be scrambled 
by introducing instead of the 3-kc carrier a 




Figure 1. Use of copper oxide varistors as modulators. In A, balanced coils are necessary; in B, coils are 
not required. 



Figure lA, consists of four copper oxide 
varistor units between two balanced coils. The 
carrier frequency is fed into the midpoints of 


square wave whose changes from positive to 
negative value are irregular in time. Each one 
of the reversals in the carrier wave causes a 


1 




















2 


SPEECH SCRAMBLING METHODS 


reversal of phase in the speech wave. The pat¬ 
tern of these irregular reversals may be ar¬ 
ranged so that the speech becomes unintel¬ 
ligible. At the receiving end a coding wave must 
be introduced which is exactly in step with the 


consists of the speech band right side up but 
displaced from its normal position by 2 kc. If 
the 16-kc value is used the output consists of 
the 3-kc speech band inverted and displaced by 
3 kc. We might use these two values alternately 
at short intervals, or we might have the carrier 



one at the receiving end with proper allowance 
for any delay there may be in the transmitting 
channel. 

A two-channel system using one modulator 
for each channel is shown in Figure 3. In this 
system the carrier fed into both modulators 
is the same in frequency but differs 90 degrees 
in phase. Two separate speech channels can be 
transmitted by this method without substantial 
mutual interference, but both sidebands as well 
as the carrier must be transmitted. At the re¬ 
ceiving end the carrier must be split into two 
components with the proper phases. Each com¬ 
ponent will demodulate its own portion of the 
signal and thereby separate the two speech 
channels. One of the channels may consist of 
noise or spurious speech from a recording or 
the like, which tends to mask the real message 
if the signal is demodulated with an ordinary 
set. This scheme was originally proposed as a 
multiplex system, but an obvious variation is 
to divide a single speech band into two halves 
with filters and then transmit the two halves 
on carriers differing by 90 degrees in phase. 


Figure 3. Split-phase multiplex used as two- 
channel system. 

vary continuously back and forth, say between 
13 and 16 kc. Another variation is to use a 
multiplicity of values, for example, 500 or 1,000 
cycles apart, (not between 10 and 13 kc for 
this illustration) and switch between these 
values in a regular or irregular sequence. A 



Figure 4. System in which signal is modulated 
twice, requiring wider frequency band than is 
usually provided by radio sets or telephone lines. 


disadvantage of these systems is that the trans¬ 
mission channel needs to be wider than that 
usually afforded by radio sets or telephone 
lines. In all these systems, the speech is restored 
by passing through identical equipment in the 
opposite direction. 


13 TRIPLE MODULATION—RE-ENTRANT 
BAND SHIFT 


12 SYSTEMS INVOLVING DOUBLE 
MODULATION 

Figure 4 shows a much more flexible system. 
Here the signal is modulated twice, with a 
band-pass filter between the two modulators. 
With this arrangement the carrier frequency 
fed into the second modulator can be varied in 
several ways. In the illustration two carrier 
frequencies are shown for the second modu¬ 
lator. If the 8-kc value is used the output 


Going back to Figure 2, suppose the carrier 
frequency were made 4 instead of 3 kc, but 
the 3-kc input and output filters were retained. 
The output would then be an inverted sideband 
ranging from 1 to 3 kc; that portion of the 
sideband above 3 kc would be cut off by the 
output filter. Since, however, there is a 1-kc 
gap at the lower edge of the transmitted band, 
the portion which would be cut off by the filter 
might be modulated down and sent along with 
the rest of the signal in this lower part of the 






























BAND-SPLITTING SYSTEMS 


3 


spectrum. In other words the portion of the output low-pass filter. A variation of this 
sideband which would otherwise disappear arrangement is to allow the 7-kc carrier to vary 
above the upper edge of the transmitted band in discrete steps according to some regular or 
might be made to reappear at the bottom. irregular program or vary it continuously be- 



Figure 5. System involving re-entrant inversion. 


Figure 5 shows a system of modulators and 
filters for accomplishing this kind of band shift. 
The first modulator is followed by a high-pass 
filter which selects the upper sideband from 
3 to 6 kc. This is combined with some of the 
original signal which ranges from zero to 3 kc. 
The second modulator is fed with a carrier 
frequency of, for example, 7 kc which inverts 
the whole band. This is followed by a band-pass 
filter passing the range from 3 to 6 kc. A third 
modulator with its carrier frequency placed 


tween the limits of 6 to 9 kc. This provides a 
variable band-shifting arrangement without 
using more than the normal 3-kc transmission 
channel. 


14 BAND-SPLITTING SYSTEMS 

A privacy system in wide commercial use, 
known as the split-band system, involves split¬ 
ting up the whole speech band into a number 



Figure 6. One form of split-band system in commercial use. 


at the lower edge of the 3- to 6-kc input band of subbands and shifting these around out of 
moves the whole band, still inverted, down to their normal positions in the frequency spec- 
the usual range of zero to 3 kc. The upper side- trum. Figure 6 shows one manner in which 
band of this modulation step is removed by the this can be accomplished. The numerical values 













































































4 SPEECH SCRAMBLING METHODS 


are chosen so that the band from 250 to 3,000 
cycles is divided into five subbands each 550 
cycles wide. 

The speech band is fed to five modulators 
in parallel. The five band filters following the 
modulators are all alike, passing the band from 
3,100 to 3,650 cycles. It will be seen that the 
uppermost modulator in Figure 6 with its 
carrier of 6.1 kc will invert the speech band 
and displace it by such an amount that the 
frequency band which originally occupied the 
space from 2,450 to 3,000 cycles will pass 
through the filter. In other words, this modula¬ 
tor in combination with its band filter selects 


An additional set of frequencies is indicated 
in the drawing for the second set of modu¬ 
lators. These frequencies will cause the output 
subbands to be inverted instead of right side 
up. One or all of these alternate frequencies 
may be used as desired. 

The '‘switch” may be changed as often as 
desired. On an experimental basis the codes 
have been changed as often as 25 times per 
second without appreciable distortion in the 
quality of the received speech, showing that it 
is possible to shift bands as wide as 550 cycles 
at a rapid rate without generating appreciable 
distortion products. 



* BP FILTERS ALL ALIKE -3000 TO 3600'>^ 

Figure' 7. Time division multiplex in which N separate signals are sent over line, each signal being 
transmitted only 1/Nth of time. 


the uppermost of the five subbands from the 
input signal. Similarly the lowest modulator 
in combination with its band filter selects the 
lowest subband from the input signal. The out¬ 
puts of the band filters all occupy the same 
frequency range, but they all came originally 
from different frequency ranges. Similarly the 
output modulators are so designed that each 
one accepts the band from 3,100 to 3,650 cycles 
and shifts it to a particular band location in 
the output. The five leads going into the box 
labeled “switch” may, therefore, be cross- 
connected in any desired manner with the five 
output leads. The resulting output will always 
cover the complete range from 250 to 3,000 
cycles and there will be no overlapping sub¬ 
bands. 


TIME DIVISION MULTIPLEX 

Time division multiplex [TDM] is a system 
in which N separate signals occupying the same 
frequency range are sent over a single line, 
each signal being transmitted only l/Ath of 
the time. This might be illustrated by showing 
the N signals connected to the N segments of 
a commutator. A rapidly rotating brush picks 
up the N signals one after the other. For 
acceptable quality, however, the brush must 
make at least as many rotations per second as 
the highest frequency in the transmitted signal. 
This means that a mechanical brush is out of 
the question and is used simply for illustration. 
This kind of switching, however, can be 
accomplished with electronic ring circuits. 




































TIME DIVISION SCRAMBLING 


5 


Since we are interested here in privacy sys¬ 
tems rather than multiplex systems, we will 
confine ourselves to the use of TDM for trans¬ 
mitting a single speech channel. This can be 
accomplished by dividing the speech band into 
a number of subbands all occupying the same 
frequency range, and connecting these to the 
segments of our hypothetical commutator. Re¬ 
ferring to Figure 7, which is similar to Figure 
6, this can be accomplished by feeding all the 
output modulators with the same carrier, and 
connecting each modulator to a commutator 
segment. In this illustration, there are four 
600-cycle subbands, covering the range from 
400 to 2,800 cycles. It has been shown mathe¬ 
matically that the output of this system 
consists of sidebands around a frequency corre¬ 
sponding to the rotation of the brush and also 
sidebands around frequencies .corresponding to 
odd harmonics of the rotation frequency. Each 
sideband, however, contains components from 
each of the subbands. It has also been shown 
that the total channel width required for good 
transmission need be no greater than that of 
the original signal. 

To increase the privacy of this system one 
of the subbands may be replaced by a band of 
noise. This can be filtered out at the receiving 
end. Obviously this system requires a high 
degree of synchronism between the two ends. 


SYSTEMS USING TAPE RECORDING 

Leaving the frequency substitution systems 
for the time being, we will introduce a device 
which permits operating on the time scale. The 
most versatile device for this purpose is the 
magnetic tape recording and reproducing sys¬ 
tem. This takes the form of a tape of magnetic 
alloy a few mils thick either run as a loop over 
pulleys or attached firmly to the perimeter of 
a disk. The recording is done by means of small 
electromagnetic pole-pieces. The signal is 
picked up by similar pole-pieces which may be 
placed at a distance from the recording pole- 
piece depending on the amount of delay desired. 
The outstanding advantage of the magnetic 
tape system for this type of application is that 
the signal may be erased and the recording 


medium be used over and over again. The 
quality of this type of transmission can be made 
very good with proper design. 

Figure 8 shows a rather simple privacy sys¬ 
tem using magnetic tape. The input signal is 
passed through a 3-way pad, whereby it is 
impressed on a band filter, and also recorded 



Figure 8. Variable subband delay system using 
magnetic tape. 


on the magnetic tape. It is picked up by equally 
spaced pole-pieces each associated with a dif¬ 
ferent band filter. With the arrangement shown 
in Figure 8 the band from 0 to 1 kc is trans¬ 
mitted without delay. The band from 1 to 2 kc 
is transmitted with 100 msec (0.1 sec) delay 
and the band from 2 to 3 kc is delayed 200 msec. 
At the receiving end the scrambled signal is 
passed through an identical system in the same 
way except that the two extreme band filters 
are interchanged. In this way the band which 
received no delay in transmission is given 
maximum delay in the receiving machine, and 
the band which received maximum delay in 
transmission is given zero delay in the receiver. 
In this way all the bands are delayed the same 
amount and the speech is restored to normal. 

This system alone does not provide any high 
degree of privacy, but it can be combined with 
other systems, as we shall see. 


1 ^ TIME DIVISION SCRAMBLING 

An important class of scrambles involving 
magnetic tape is known as time division 
scrambling [TDS]. A simplified diagram of this 
system is shown in Figure 9. There are a re¬ 
cording pole-piece and a number of pickup pole- 
pieces. There is also a commutator driven in 
synchronism with the tape. The length (in 
time) of each segment of the commutator is. 



















6 


SPEECH SCRAMBLING METHODS 


in general, equal to the delay between succes¬ 
sive pickup pole-pieces. However, the number 
of segments need not be the same as the number 
of pole-pieces. A switch is provided whereby 
any segment may be connected to any pole- 
piece. 



Figure 9. One form of time division scrambling 
[TDS]. 


With this system the speech is cut up into 
time elements corresponding in length to the 
spacing of the pole-pieces. These time elements 
are transmitted in a scrambled order. For 
instance, six successive time elements which we 
might label 1, 2, 3, 4, 5, 6 might be transmitted 
in the order 2, 4, 1, 3, 6, 5. The possibilities 
of TDS coding are far too complex to cover 
here. The general requirements for all TDS 
systems may be stated as follows: (1) Each 
element of the original speech must be trans¬ 
mitted once and only once. (2) The sum of 
the delay in the transmitting machine plus 
the delay in the receiving machine must be 
equal for all elements. With these two require¬ 
ments fulfilled it is obvious that the speech 
comes out of the receiving machine in its 
normal order. It is delayed, however, by an 
amount equal to the sum of the transmitting 
and receiving delay. 

At the receiving end there are several ways 
of handling the scrambled signal. (1) The pick¬ 
up pole-pieces can be used as recording pole- 
pieces and the signal picked up by an additional 
pole-piece shown at the right in Figure 9. 
With this arrangement the connections between 
the commutator and the pole-pieces are the 
same in the transmitting and receiving ma¬ 
chines. (2) The signal can be recorded with 
the same pole-piece used in the transmitting 


machine and the connections between the pole- 
pieces and the segments rearranged for re¬ 
ceiving by a push-to-talk relay. (3) The codes 
can be restricted to a particular class called 
self-converse codes. These have the property of 
being self-decoding, that is, the same code 
which scrambles the speech in the transmitter 
restores it in the receiver. 


18 INTERLACE 

An important variation of this system is 
called “interlace.” In this system the number 
of segments on the commutator is doubled. 
The odd segments are connected to the pole- 
pieces according to one code and the even 
segments are connected according to a com¬ 
pletely independent code. The reason for this 
device is to increase the difficulty encountered 
by the enemy in trying one code after the 
other to find the right one, particularly if the 
total number of codes available is small. With 
the interlace system the total number of com¬ 
binations possible is equal to the square of the 
number of codes. 



Figure 10. Speed wobble in which speech time 
scale is alternately compressed and expanded. 


The rotating commutator shown in Figure 
9 results in a repeated code, that is, each rota¬ 
tion produces the same scramble. It is possible 
to substitute for the commutator and switch 
arrangement, shown in Figure 9, a more com¬ 
plex arrangement whereby the speech is 
scrambled in a never repeating manner. There 





























COMBINATIONS OF TIME AND FREQUENCY SCRAMBLING 


7 


are several ways of accomplishing this. Perhaps 
the simplest way to represent it is as a punched 
tape which permits the pole-pieces to be con¬ 
nected to the output, one at a time, in any 
desired order permissible under the restrictions 
outlined above. 

Another way of utilizing magnetic tape to 
scramble speech is shown in Figure 10. Here 
the pickup pole-piece is oscillated back and 
forth along the tape mechanically. With this 
arrangement, or other variations equivalent to 
speech changes, the speech time scale is alter¬ 
nately compressed and expanded. The fre¬ 
quency scale is correspondingly expanded and 
compressed, respectively. 

With the arrangement shown in Figure 11, 
speech is broken up into time segments each 



IM * SAME DIRECTION AS TAPE 

- _ BUT TWICE AS FAST 

Figure 11. Time inversion in which speech is 
broken up into time segments, each of which is 
transmitted backwards. 

of which is transmitted backwards. The motion 
of the pickup pole-pieces is twice as great as 
the motion of the tape and is in the same 
direction. Therefore, the relative motion of the 
tape and the pole-pieces is the reverse of that 
used in recording. This is the same as running 
the tape backwards for reproduction. 


' ^ COMBINATIONS OF TIME AND 
FREQUENCY SCRAMBLING 

Obviously the two kinds of systems described 
in the previous sections can be used together. 
For instance, some of the time elements of a 
TDS system might be inverted according to 
a regular or irregular program. The next more 
complex step is to combine the band-splitting 
system of Figure 6 with the TDS system. The 


codes of the band-splitting system might be 
fixed or might be switched in synchronism with 



Figure 12. Multiplication system in which speech 
wave is multiplied by coding wave. 


the TDS elements, the time scale of the 
scrambled speech not being further broken up. 
If they are switched nonsynchronously, how¬ 
ever, the time dimensions will be further broken 
up, as will be seen subsequently. Combinations 
of nonrepeated code TDS and rapidly switched 
split-band coding can be made to afford a very 
high degree of privacy. The two kinds of 
coding, of course, must not be so interrelated 
that one furnishes clues for the other. If, for 
instance, a certain pole-piece were syste¬ 
matically associated with a certain split-band 
code the total privacy of the combination might 
be impaired rather than enhanced. A coding 
method for avoiding this difficulty is described 
in Preliminary Report^ No. 21 of Project C-43. 

A very special kind of scramble is produced 
by a system which consists functionally of 
Figure 6 (rapidly switched) in tandem with 
Figure 8 (with five bands) followed by an 
additional Figure 6. This is not the simplest 
form of the system, but it serves to illustrate 
the principle. Two frequency scrambles with 
a time shift in between produce a particular 
kind of two-dimensional scramble in which the 
speech is broken up into both time and fre¬ 
quency elements. Each of these elements may 
be shifted both in time and in frequency so 
as to be out of proximity with other elements 
with which they were originally associated 
either in time or in frequency. Another way 
of accomplishing this kind of scramble would 
be a combination of rapidly switched split band 
with a separate TDS system in each subband. 
A two-dimensional system was described in 
the Brown Boveri article^ reproduced in Pre¬ 
liminary Report No. 5^ and analyzed in Pre¬ 
liminary Report No. 9-^ of Project C-43. 

For the sake of completeness two other sys- 

















8 


SPEECH SCRAMBLING METHODS 


terns involving time and frequency shifting will 
be mentioned, although as far as is known they 
exist only on paper. Suppose a sample of speech 
were recorded on tape and then reproduced 
at twice the normal speed. It would occupy only 
half the time it took to speak the words, but 



its frequency range would be twice the normal 
range. Let the upper half of the expanded 
frequency range be separated by a filter and 
modulated down to the normal range and used 
to fill up the unused time. The directly opposite 
but analogous system would involve reproduc¬ 
ing recorded speech at half its normal speed; 
the frequency range would then be only half the 
normal range. Alternate sections, therefore. 


could be modulated up to fill the unused fre¬ 
quency space, thereby keeping the total trans¬ 
mitting time substantially unchanged. In both 
of these systems, there would be a delay equal 
to the length of one time element. 


1.10 WAVE FORM MODIFICATION 

Thus far we have considered systems in 
which frequency bands were shifted around 
or time elements were rearranged. There are 
a few privacy systems which make speech 
unintelligible by a direct modification of the 
wave form. One of these is shown diagram- 
matically in Figure 12. It depends upon a 
process whereby two waves are multiplied 
together, that is, the instantaneous amplitude 
of the resulting wave is the product of the 
amplitudes of the two input waves (not the 
sum or the difference as is the case in the simple 
inversion methods described above). One of 
the input waves to the multiplier is speech. 
The other is a complex coding wave. If the 
coding wave is sufficiently complex the result¬ 
ing scramble is unintelligible. At the receiving 
end a reciprocal of the coding wave is derived 
and used as a multiplier, thereby restoring the 
original speech. Naturally, the coding waves 
at the two ends of the system must be in close 
agreement, otherwise there will be considerable 



Figure 14. Level modulation; form of wave form 
alteration in which drastic changes in speech 
levels occur. 


background noise in the decoded speech. An 
example of the multiplication system is shown 
in Figure 13. 

Another method for changing the wave form 
is shown in Figure 14. The essential feature 
of this system is an amplifier whose gain can 
be varied rapidly with time. Drastic changes 





























MASKING SYSTEMS 


9 


in the level of speech, if they occur rapidly 
enough, will make the speech unintelligible. The 
level changes might be made according to some 
program or they might be made to follow the 



CONTROL CKTS 


Figure 15. Level modulation system in which 
speech is first divided into subbands, each of 
which is subjected to level changes. 

speech wave itself. For instance, extreme com¬ 
pression or expansion could be used. Corre¬ 
sponding gain changes, of course, must be made 
at the receiving end. 

A variation of this system is shown in Figure 
15. Here the speech band is first divided into 
subbands, and these are individually subjected 
to level changes according to separate pro¬ 
grams. 


use very high levels of masking signals to hide 
the intelligibility. This makes it difficult to 
subtract out satisfactorily: the difficulties are 
such that masking systems are more likely to 
be found on wire lines than on radio. A few 
speculative masking systems are outlined in 
the following paragraphs. 

One form of masking system is shown in 
Figure 16. In this system, two telephone lines 
are used. At the sending end, noise is added 
to the speech in a mixing pad and the combina¬ 
tion is sent over line 1. The noise alone is sent 
over a second line and is used at the receiving 
end to cancel the noise transmitted with the 
speech by simple subtraction. This system has 
the advantage that the noise can be completely 
random. However, since the enemy might tap 
both lines and thereby be able to make the 
same subtraction, a variation of this system 
consists in distorting the noise in some pre-. 
determined manner before sending it over the 
second line. At the receiving end, this distor¬ 
tion is first nullified so that the noise may be 
subtracted. Naturally, the form of distortion 
must be unknown to the enemy. It can, of 
course, be varied from moment to moment. 

Another masking system is shown in Figure 
17, which uses only one line. In this system, 
noise is added to the line at the receiving end 
instead of at the sending end. Again, the noise 
can be perfectly random. Since the noise is 
generated at the receiving end, the process of 



Figure 16 . Noise masking requiring use of two telephone lines. 


1MASKING SYSTEMS 

One of the first schemes which is likely to 
occur to a person considering how to make 
speech private is to add noise or other disturb¬ 
ing signal to the speech and remove it at the 
other end, in other words, to mask the speech. 
He will find, however, that it is necessary to 


cancellation can, theoretically, be made very 
exact. This system, however, cannot be used for 
radio at all because the level of the noise de¬ 
creases with distance from the receiver, while 
the level of the signal increases. The inter¬ 
ceptor, therefore, will get good speech signals 
if close to the transmitter. With telephone lines 
this differential can be kept small. 















































10 


SPEECH SCRAMBLING METHODS 


Another simple masking system is to have 
a sequence of tones superposed on the signal 
at the transmitting end. At the receiving end, 



Figure 17. Method of applying masking noise at 
receiving end of communication circuit. 

sharply tuned band elimination networks can 
be synchronously switched so as to remove the 
tones from the listener’s ear. Similarly, short 


can be made to occur at irregular intervals 
according to a never repeating program. Both 
of these systems involve the loss of small por¬ 
tions of the speech, either in the time scale or 
the frequency scale. 

A system described in Preliminary Report 
No. 4 of Project C-43 might be classified as 
a masking system, although it might be better 
classified as a means of communicating without 
the enemy’s knowledge. 


112 VOCODER SYSTEMS 

The Vocoder system®*^ may be made the 
basis for privacy systems of various kinds. The 



_f 


r4 









SPEECH 

OUTPUT 


Figure 18. Vocoder system which may be made basis of privacy systems of various kinds. 

spurts of noise covering the whole frequency system is shown schematically in Figure 18. 
band can be applied at the transmitting end At the transmitting end the speech is passed 
and shorted out at the receiving end. The spurts through a series of band filters, the outputs 




































































































CHANNEL-MIXING SYSTEMS 


11 


of which are individually rectified to form a 
fluctuating d-c signal. These signals are indi¬ 
vidually modulated in such a way that they can 
all be sent over a single transmission path. 

At the receiving end synthetic speech is 
manufactured in accordance with the signals 
transmitted over the line. A source of noise 
which covers the whole frequency range is 
passed through a set of band filters similar to 
those at the transmitting end. The output of 
each of these filters is controlled so that it is 
the same level as the level of the speech in the 
corresponding band at the transmitting end. 
This is accomplished by separating the signals 
in the various channels, detecting them and 
using the resulting fluctuating direct current 
to control the variable gain amplifiers in their 
respective channels. 

The noise is of two types, depending on 
whether a voiced or unvoiced sound is to be 
simulated. For an unvoiced sound, it is a hiss¬ 
like thermal noise. For a voiced sound it is 
a buzz which consists of a series of harmonics 
covering the whole frequency range. A separate 
carrier is used to transmit information for 
operating this part of the system. At the trans¬ 
mitting end the pitch used by the talker is 
measured and this information is used to 
control the pitch of the buzz sound. The absence 
of a pitch signal switches the hiss sound into 
the system. 

This system by itself, of course, is not pri¬ 
vate, since the enemy can build a similar system 
and use the signals to regenerate speech. 
Privacy must be achieved by operating on the 
channel signals. One method is to permute the 
channels at short intervals according to a 
prearranged program. Another method is to 
put a TDS system into the line, or into each 
channel separately. A still more effective 
method of this type is to apply a two-dimen¬ 
sional scramble, such as was described earlier, 
to the channels so that signal elements are 
displaced in both time and frequency. 


CHANNEL-MIXING SYSTEMS 

Thus far, the methods we have examined 
apply to a single transmission path. There is 


another class of privacy system which depends 
on using a multiplicity of paths. This is, of 
course, inefficient if only a single message is 
to be transmitted. However, the method can 
be applied to cases where a number of channels 
exist between two points and a number of mes¬ 
sages would normally be transmitted over these 
channels simultaneously. 



Figure 19. Channel mixing in which multiplicity 
of paths is involved. 

Figure 19 shows one form of channel-mixing 
system. Here three channels are shown 
connected to the three segments of a com¬ 
mutator. Three brushes on this commutator are 



Figure 20.. Subband channel mixing. 

connected to the outgoing channels which are 
thereby caused to pick up one channel after the 
other on a time division basis. Each channel 



































12 


SPEECH SCRAMBLING METHODS 


contains parts of messages from all three 
channels. The commutator, of course, is too 
simple to be very effective and would, in prac¬ 
tice, be replaced by a permuting switch capable 
of switching according to a more complex pro¬ 
gram. One or more of the channels may be 
filled up with the noise or spurious speech from 
a recording or other similar source. 

An analogous system which divides the mes¬ 
sages on a frequency basis is shown in Figure 
20. Here each channel is passed through three 
band filters which divide the speech into sub¬ 
bands. Each of the outgoing channels contains 
subbands from each of the incoming channels. 
To increase the privacy, a permuting switch 


is shown which rearranges the subbands on a 
time division basis. If only one message is to 
be transmitted the other channels can be filled 
in with noise or spurious speech. 

SUMMARY 

The above examples cover fairly completely 
the range of schemes that might be used to 
scramble speech at audio frequencies. In subse¬ 
quent chapters we will examine each system 
from the decoding standpoint. To facilitate 
reference to the various systems, they are 
summarized in Chapter 4, Table 1. 



Chapter 2 

TIME DIVISION SCRAMBLING SYSTEMS 


INTRODUCTION 

T he two main divisions or characteristics 
of speech are the frequency dimension and 
the time dimension. Either or both of these 
components of speech may be rearranged or 
altered before or during transmission to insure 
privacy. At the receiver, the order of scram¬ 
bling is reversed so that the original sense is 
recovered. 

A privacy system developed to a high order 
during the war, known as time division scram¬ 
bling [TDS], operates on the time character¬ 
istics of speech. In this system successive 
sections of speech, each m seconds long, are 
divided into n short time elements, and these 
n elements are sent in a scrambled time 
sequence. The elements are much shorter than 
a syllable, so that each word is cut up and 
received as short bursts of energy in the wrong 
order. 

All the summaries which follow deal with 
TDS systems. 


' 2 PORTABLE TDS SYSTEMS—PROJECTS 
C-I AND C-IA 

While several forms of speech scramblers 
had been in use on radio circuits before 1940, 
there was none which was small and light 
enough to be suitable for mobile warfare. Work 
done in the summer of 1940 at the Bell Tele¬ 
phone Laboratories indicated that the need 
might be met in the form of time division 
scrambling by a small magnetic-tape recorder. 
The purpose of Projects C-1 and C-IA, there¬ 
fore, was to investigate the possibilities of pro¬ 
ducing such a lightweight and effective privacy 
unit based on the TDS principle. 

The fundamentals of this scheme were not 
new. It was known that such privacy should 
be very effective without appreciable expansion 
of the original frequency band, but the idea 
had not been developed for two reasons. First, 


it could not be used for commercial telephony 
because scrambling in time requires that the 
speech be stored for a certain time which 
introduced more delay than can be tolerated 
by inexperienced users. Secondly, there were 
several difficult technical problems that had 
not been solved. 

Design ideas were collected by discussions 
with various specialists and a particular ar¬ 
rangement was visualized before the work was 
started. This set an objective and helped in 
segregating several problems that could be 
handled more or less individually and simul¬ 
taneously. The more important of these in¬ 
cluded : 

1. Development of a way to mount magnetic 
tape on the edge of a disk without introducing 
serious magnetic irregularities. 

2. Development of a start-stop commutating 
arrangement for rapidly switching magnetic 
tape recorders and reproducers. 

3. Development of a much more stable 24- 
volt motor drive than was then available. 

4. Development of a compact amplifier unit 
to meet the special requirements of this system. 

5. Development of a suitable switching ar¬ 
rangement for setting up scrambled combina¬ 
tions. 

As the project progressed, the original de¬ 
sign was changed where advisable and details 
were added. The five months set for completion 
of the project made it necessary to avoid any 
suggested changes that would appreciably 
delay the construction of models. 

While development of experimental equip¬ 
ment was in progress, a separate group was 
investigating how various design factors would 
affect possible requirements for future equip¬ 
ment. These investigations were of two sorts. 
The first was concerned with the degree of 
privacy afforded by the equipment, and the 
second with factors affecting the final quality 
of the speech. 

The degree of privacy was investigated as 
a function of the number of scrambling inter¬ 
vals within a cycle and the length of a cycle. 


13 




14 


TIME DIVISION SCRAMBLING SYSTEMS 


In this connection a rather thorough study was 
made of methods to “crack” the TDS privacy 
under conditions ranging from limited facilities 
and personnel to extensive laboratory equip¬ 
ment handled by experts. The conclusions were 
that cracking was rather unlikely unless 
attempted by experts with special equipment, 
and in this latter case the five-unit design 
developed for experimental use might be 


elements associated with the scrambling and 
reconstruction process affected the final quality 
of the speech. The most important conclusion 
here was that the degree of stability afforded 
by the special motor designed for the experi¬ 
mental TDS unit met the requirements very 
satisfactorily. The indications were that it 
would not be difficult to provide a final design 
which would introduce little degradation. 



DYNAMOTOR 


CODING DEVtGE 

BHBBtransfer relays 


MOTOR 




FILTER 


AMPLIFIER 


GAS TUBE AND CONDENSER 


Figure 1. General view of TDS system developed under Project C-1. 


cracked within minutes. In spite of this, it was 
believed that the five-unit design would be very 
effective when used on radio telephone circuits 
for the direction of maneuvers that are com¬ 
pleted within minutes. This would suggest use 
for such links as plane-to-plane, plane-to- 
ground, and tank-to-tank. 

The investigations concerned with speech 
quality included such things as the way in 
which accuracy of synchronism, start-stop 
brush operation, and the position of various 


With the experimental TDS unit produced 
during the project about fifty useful combina¬ 
tions could be set up on the five front-panel dial 
switches. When used in association with radio¬ 
telephone equipment, connections were required 
to the transmitter, receiver, and power supply. 
The system used 100 watts at 24 volts, but 
would operate over a voltage range of approxi¬ 
mately 22 to 30. The weight was about 40 lb 
and the dimensions were 7% in* high, 9 in. 
deep, and 20 in. long. 







PORTABLE TDS SYSTEMS—PROJECTS C-l AND C-IA 


15 


Demonstrations 

In March and April 1941, four experimental 
TDS units were demonstrated in Washington 
before members of the National Defense Re¬ 
search Committee [NDRC] and representatives 
of the Army and Navy. The units were set up 


of a drum or wheel on the periphery of which 
is mounted a wide magnetic tape alloy. Around 
the wheel and in contact with the tape are 
eleven pole-pieces, mounted so that all traverse 
the same narrow band of the tape. The pole- 
pieces comprise one eraser supplied with direct 
current for magnetically saturating the tape to 



Figure 2. Tape wheel of C-l TDS unit showing placement of individual reproducers, etc. 


first in separate rooms at the Carnegie Insti¬ 
tution and later one unit was set up in the Navy 
Building and another in the Munitions Building 
with connections to telephone extensions in 
both places. The equipment was demonstrated 
to representatives of the Army and Navy as 
well as to members of the British Military 
Mission and the Canadian Air Ministry. 

" " " The TDS Units 

The essential part of the TDS units consists 
of the magnetic recorder-reproducer made up 


obliterate previous recordings, one recorder 
supplied with speech current, and a 1-ma d-c 
depolarizing current and nine reproducer pole- 
pieces spaced 36 degrees apart. 

Associated with each reproducer pole-piece is 
a segment of a commutator and a contact to a 
switching system so that the order in which 
the recorded speech currents are taken off for 
transmission by radio or wire can be changed 
from the order in which the recording was 
made. 

The intervals of speech each 0.30 sec long 
are divided into five sections of 0.06 sec dura- 









16 


TIME DIVISION SCRAMBLING SYSTEMS 



LATCH COIL 


CLUTCH FELTS 


BRUSH ARM 
EXTENSION 


ARM PIVOT 


LATCH 


COMMUTATOR 

SEGMENTS 


ARM BACK STOP 


CLUTCH 

ADJUSTMENTS 


SLIP 


RING 


BRUSHES 


PULSE 1 
CONTACTS 


BRUSH ARM 
EXTENSION 


PULSING 


CAM 


Figure 3. Commutator details. 


tion. The instrument is described in consider¬ 
able detail in the final reports of the project. 

The final report^’of Project C-IA, a con¬ 
tinuation of Project C-1, describes methods of 
cracking the TDS system, gives an estimate of 
the time required to crack it, an evaluation of 
the privacy secured by it, and recommendations 


concerning future developments, some of which 
resulted in Projects C-50 and C-65. 

Conclusions 

The project demonstrated that TDS could 
provide a useful degree of privacy in a portable 
















CONTINUOUSLY CODED TDS—PROJECT C-SO 


17 


device of reasonably small dimensions and 
weight. Further developments to meet military 
needs were pursued by the Services themselves. 
The problem of speech privacy was attacked 
in other NDRC projects along many other lines, 
as evidenced by summaries of other Division 13 
projects in this volume, but to the end of the 
war TDS remained the only small and portable 


equipment by provisions for automatic code 
changing every code cycle. In the winter of 
1942 means for doing this were suggested and 
Project C-5010 was set up to develop these 
means. The specific object was to provide model 
equipment so that the privacy obtained by 
elaborating the TDS principle to its practical 
limit could be studied. 



Figure 4. Simplified circuit diagram of C-1 TDS system. 


device for scrambling speech that was in fac¬ 
tory production. 


2 3 CONTINUOUSLY CODED TDS— 
PROJECT C-50 

The report on Project C-1 A suggested an 
improvement of security provided by TDS 


In the manually operated TDS system, the 
nine pole-pieces which “take off’^ the recorded 
segments of speech are connected to a commu¬ 
tator and switch by which the operator can 
change the order in which the successive ele¬ 
ments are transmitted to the lines or to a radio 
transmitter. The object in using different ones 
of the various pole-pieces is to retard or delay 
successive elements of the recorded message or 

































































































































18 


TIME DIVISION SCRAMBLING SYSTEMS 


speech by different amounts. When two speech 
elements which were originally in succession 
are delayed by different amounts they are dis¬ 
placed in time with respect to each other. They 
may be merely interchanged with respect to 
each other or they may be separated widely by 
inserting other speech elements between them. 

The scrambled speech elements are returned 
to their proper order in the unscrambling proc- 


cording pole-piece, and an erasing pole-piece. 
In the C-50 TDS system, each pole-piece is 
capable of recording, of erasing, and of repro¬ 
ducing. Such a system is known as a ten-ele¬ 
ment system because ten speech elements are 
scrambled in each code cycle. In the C-50 sys¬ 
tem, two interlaced ten-element codes are em¬ 
ployed in each code cycle. 

If the interconnection of pole-pieces and 



Coils begcbhb hgf 

Figure 5. TDS coding connections. Transmit machine codes on “reproduce,” while receive machine decodes 
on “record.” 


ess by delaying each one in a selective manner 
so that the total delay contributed by scram¬ 
bling and unscrambling is the same for each 
element. Elements scrambled with minimum 
delay are unscrambled with maximum delay, 
and vice versa. The speech, when reassembled, 
has been delayed by the sum of the minimum 
and maximum delays. In the C-50 TDS system 
this total delay is 700 msec. 

^ The C-50 TDS System 

In the TDS units developed under Project 
C-1, there were nine take-off pole-pieces, a re¬ 


commutator segments remains fixed, the code 
or key is fixed and the transposition scheme is 
repeated with each revolution of the brush over 
the twenty commutator segments. This takes 
place in 750 msec. By means of punched code 
cards, the C-50 TDS system can be operated in 
this manner, if desired. 

Continuous Coding 

The continuous coding equipment makes two 
changes in the interconnection of pole-pieces to 
segments in each revolution. The connections 
to odd-numbered segments remain fixed until 






































































































CONTINUOUSLY CODED TDS—PROJECT C-50 


19 


the commutator brush leaves the nineteenth 
segment; when the brush starts on segment 
No. 1 an entirely different set of interconnec¬ 
tions is used in the next revolution by the odd- 
numbered segments. The pattern for the even- 
numbered segments begins with the twelfth 
segment and extends around the commutator 
through the tenth segment; when the brush 


required for the magnetic tape to move from 
one reproducing pole-piece to the next. The 
portions of speech available for reproduction 
when the brush is on odd segments are, there¬ 
fore, never available when the brush is on even 
segments. 

The patterns of interconnection between pole- 
pieces and commutator segments must follow 



MAGNETIC 

TAPE 


DELAY INDICATOR 
AND CYCLING 
SEGMENT transfer 
GEAR BOX-- 


COM. ADJ. SHAFT 


Figure 6. Details of C-50 TDS machine. 


reaches the twelfth segment again a new pat¬ 
tern is ready for the even segments. In this way 
the coding for alternate segments constitutes 
two independent systems of transpositions, each 
changing every 750 msec but displaced from 
each other by 375 msec. 

The odd-numbered and the even-numbered 
commutator segments can be treated separately 
because the time taken for the brush to cover 
one segment, 37.5 msec, is exactly half the time 


certain rules in order that the TDS generate a 
transposition which is valid, in the sense that 
each speech element is transmitted once and 
only once. Since speech on the magnetic tape 
remains there as it goes past all nine reproduc¬ 
ing pole-pieces, a random choice of reproducing 
pole-pieces might pick up the same element of 
speech twice or more, and some other element 
would be omitted for each repetition. 

The kinds of interconnections which give 








20 


TIME DIVISION SCRAMBLING SYSTEMS 


useful codes are discussed in Appendix A of 
the project final report. The rules worked out 
in that appendix are basic in the design of the 
automatic coding apparatus which must be so 
arranged that the semirandom series of choices 
called for in one part of the coding equipment 
is scrutinized in another part and revised until 
the interconnection finally set up is a valid 
transposition. 


^ Number of Codes Available 

The complexity of the coding systems gen¬ 
erated by the automatic coding equipment in 
the C-50 TDS may be illustrated by the fact 
that there are 1,625,702,400 dilferent sequences 
of codes for each of the two interlaced systems 
or the square of this figure for the combination. 
The choice of the particular sequence to be 
used is governed by four punched cards. The 
initial settings of ten selector switches then 
determine the point in the sequence at which 
the sequence will start. There are (3,282,972 
such starting points for each sequence. 

From the point of view of those operating 
the equipment, the four punched cards and the 
ten selector switch settings constitute the key, 
since these choices must be agreed upon at both 
ends of the circuit. 

When the cards and initial settings have been 
chosen, the equipment can be started, after 
which it provides in each of the interlaced sys¬ 
tems, an irregular order of valid codes from 
a possible number of 60,316 codes. In a single 
message, or even in a whole day’s use with the 
same initial settings, only an extremely small 
fraction of any code cycle would ever be used. 
Each particular sequence runs so long that it 
would not begin to repeat before 6,400,000 
years. 


" C-50 TDS Plus Frequency-Band 

Switching 

Although to the end of the project, decoding 
methods developed under other Division 13 
projects failed to crack the C-50 system, it was 
found that some trained individuals under fa¬ 
vorable circumstances were able to understand 
some of the scrambled speech. Less is under¬ 


stood with fast talkers than with slow talkers 
but the amount understood by such trained ob¬ 
servers at normal speech rates leads one to the 
conclusion that in a system intended to provide 
long-term privacy the TDS principle should not 
be used singly but in combination with other 
principles. 

A combined system should be so devised that 
direct listening is of no value and should be 
arranged so that an interceptor can not dis¬ 
entangle the two types of scramble and obtain 
intelligence by listening directly to one of them. 
Such a system is provided by the C-50 TDS 
combined with and controlling a rapidly 
switched frequency-band shift system (A3) so 
arranged that the C-50 TDS coding equipment 
controls the sequence of A3 codes. In this ar¬ 
rangement the A3 codes may be switched as 
often as every 37.5 msec. No useful intelligence 
had been extracted from the resultant scramble 
in direct listening tests up to the time the final 
report on Project C-50 was written. Further¬ 
more the difficulty of decoding the scrambled 
speech by other methods was expected to be 
materially increased. Although the combined 
C-50 TDS and A3 systems weighed about 2,200 
lb and required a power of 1,500 watts, its 
probable privacy and its availability were such 
as to be recommended for use in corps-to- 
division communications and in similar Navy 
situations. This double scramble system is de¬ 
scribed in Appendix F of the final report of 
Project C-50. Actual work on such a system was 
carried out under Project C-66. 


^ Appendices to Report of Project C-50 

The following titles of the several appendices 
to the final report of Project C-50 give an indi¬ 
cation of the material to be found in the report 
itself but not summarized here. 

Appendix A Basic TDS Principles 

Automatic Code Generating 
Number of Codes and Code Se¬ 
quences 

Appendix D Detailed Description of C-50 
Equipment 

Operating Instructions 
C-50 and A3 Systems Combined 
Project C-50 Drawings 


Appendix B 
Appendix C 


Appendix E 
Appendix F 
Appendix G 






FREQUENCY-TIME DIVISION SYSTEM—PROJECT C-66 


21 


FREQUENCY-TIME DIVISION SYSTEM- 
PROJECT C-66 

In 1942 it was known that the speech security 
provided by the TDS units developed under 
Project C-1 was short unless automatic coding 
was used as developed in Project C-50. It was 
known, too, that only short-time security was 
provided by the A3 scramble based on permuta¬ 
tions of frequency bands. There was reason to 
believe that A3 in tandem with TDS might pro¬ 
vide a much higher order of security and 
Project C-66^1 was set up to study this. 


^ ^ ^ Accomplishments of the Project 

It was found that A3, switched even as often 
as every 60 msec, in tandem with the portable 
fixed-code TDS, gave an increase in privacy in¬ 
commensurate with the circuit complexity. 
When rapidly switched A3 was combined with 
the continuously recoded TDS of Project C-50, 
however, the degree of security attained ap¬ 
peared to justify recommendation of the devel¬ 
opment of the combination for truck-mounted 
use in corps-to-division communication. So far 
as is known, this recommendation was not acted 
upon. 

The actual security and the transmission 
features of the combined systems may be sum¬ 
marized as follows: 

1. Although it is possible under some circum¬ 
stances for trained observers to obtain some 
intelligence from listening directly to the scram¬ 
bled speech from either A3 or C-50 systems 
alone, when these are combined practically no 
intelligence can be obtained from the scram¬ 
bled speech in this manner. 

2. The unauthorized agent would require spe¬ 
cial and complex analyzing equipment to restore 
the scrambled speech from the combined sys¬ 
tems. The cracking time had not been deter¬ 
mined on the date of the final report (May 29, 
1943). The C-50 TDS alone using nonrepeating 
codes had been found very difficult to crack in a 
form permitting reproduction of the message, 
and it was the opinion of the personnel of 
Project C-43 (which was largely concerned with 


cracking methods and which became expert in 
this technique) that the cracking of this TDS in 
combination with and controlling the A3 would 
be much more difficult, since the A3 codes ob¬ 
scure the matching of the speech elements. 
Even if this can be done successfully there ap¬ 
pears to be a major development problem with 
respect to reproducing the message from the 
reassembled photographic traces. 

3. The C-50 TDS plus A3 provides transmis¬ 
sion quality of a useful grade. Under limiting 
conditions, however, the signal-to-noise ratio 
must be about 7.5 db greater than that required 
when the privacy system is not used. Under 
average conditions articulation tests showed 
that the C-50 TDS plus A3 system was equiva¬ 
lent to a reference circuit without privacy hav¬ 
ing a bandwidth from 250 to 3,000 cycles and a 
signal-to-noise ratio of about 14 db. 


Other Studies under Project C-66 

The transmission quality of devices combin¬ 
ing frequency scrambling with time delays pro¬ 
vided by means of magnetic tape was found in 
this investigation to depend upon control of the 
following factors: 

1. Modulation products giving rise to non¬ 
linearity. 

2. Flutter from speed variation in magnetic 
tapes. 

3. The signal-to-noise ratio of the overall sys¬ 
tem. 

4. The transmission frequency characteris¬ 
tics. 

5. Switching effects. 

Detailed analyses of these controlling factors 
are found in the final report of the project. 

Among other systems and schemes for ob¬ 
taining privacy by time and frequency scram¬ 
bling considered in the course of Project C-66, 
the following deserve mention: 

1. A re-entrant frequency band shifter. 

2. A two-dimensional scrambler employing 
frequency band delay plus a re-entrant fre¬ 
quency band shifter plus frequency band delays. 

3. A two-dimensional scrambler employing a 
re-entrant frequency band shifter plus fre- 






22 


TIME DIVISION SCRAMBLING SYSTEMS 


quency band delays plus a re-entrant band 
shifter. 

4. A two-dimensional scrambler employing 
A3 privacy and five magnetic-tape delay circuits 
of different delay values. 


tion expires. If a mission of mobile units takes a 
greater time than this protected period, changes 
in the code should be made during the mission. 
Only enough codes should be available to afford 
the desired protection. More codes than neces- 



Figure 7. Exterior view of Model B code-changing unit attached to D-150285 TDS unit. 


2 5 CODE-CHANGING ATTACHMENT 

FOR THE C-I TDS UNIT—PROJECT C-65 

The final report of Project C-IA suggested 
that the security of the TDS speech scramblers 
would be improved by frequent changes of code 
cards. In 1942 a variety of means had been sug¬ 
gested for accomplishing this by an attachment 
to the TDS unit (D-150285) then being pro¬ 
duced for Army and Navy. Project 0-65^2 had 
as its purpose the development of such an at¬ 
tachment. The background for this work is as 
follows: 

Since the security afforded by fixed-code pri¬ 
vacy systems cannot be relied upon beyond the 
period required by an unauthorized intercepter 
to work out the code, it is desirable to change 
the code before this minimum period of protec- 


sary would provide the enemy with too much 
information if a unit were captured. 

Under the stress of the circumstances at¬ 
tending an actual mission, the changing of codes 
might be neglected unless the operation is fully 
or partially automatic. If only partially auto¬ 
matic, the operation should require only simple 
and positive motions, such as pushing buttons 
or pulling levers, to be carried out on command. 

Two designs were selected from the many 
considered and two units each of these designs 
have been constructed. One design provides for 
electrical operation under control of a timer, 
though manual operation is possible; the other 
design is arranged for manual operation but 
could be converted to automatic operation by 
further development. The correctness of the 
operating principles has been checked by tests 











TELEGRAPHY APPLIED TO TDS—PROJECT €-55 


23 


on TDS units. Choice between the two will de¬ 
pend on military requirements and manufactur¬ 
ing considerations. 

The two models are functionally equivalent 
since each provides a choice of twenty double 
codes, and the particular sets of twenty from 
the several hundred available may be changed 
between missions. In the automatic model, codes 
are set up by means of ninety small relays, con¬ 
trolled by contacts made through two perforated 



Figure 8. Model C code-changing unit with case 
removed; back and top view. 


sheets, similar to player-piano rolls, with twenty 
codes per sheet. In the manually operated model, 
a mechanism pushes two code cards, out of two 
groups of twenty contained in loading boxes, to 
close two sets of spring contacts according to 
the arrangement of perforations in the code 
cards. 

The automatic model operates on 24 volts, 
drawing 0.8 amp between changes and 1.5 amp 
during changes. 

The automatic model is about 10 per cent 
less than the TDS unit in cubic contents, and, 
for comparable materials, of about the same 
weight. The manual model has about half the 


volume and, for comparable materials, about 
half the weight of the TDS unit. Both models 
could be operated by a single motion of a gloved 
hand. 


2 6 telegraphy applied to TDS— 

PROJECT C.55 

Project C-55^^ was undertaken at the request 
of the Signal Corps early in 1942 to determine 
the advantages and disadvantages (if any) of 
incorporating means for tone telegraphy in con¬ 
nection with TDS equipment which had been 
developed for the Services. At the same time 
the question was raised as to whether the use of 
telegraph might not jeopardize the privacy of 
the device for speech. This was based on the 
idea that the TDS code might be recovered more 
quickly from scrambled telegraph signals than 
from scrambled speech signals. It was impor¬ 
tant to answer this question to determine 
whether restrictions would be needed on the 
use of telegraph. 

The questions to be answered may be sum¬ 
marized as follows. 

1. Is TDS privacy less for telegraph than for 
speech? That is, will the use of telegraph 



Figure 9. Model B die for punching code masks. 

through TDS tend to expose the code and thus 
reduce the privacy for subsequent telephone 
use? 

2. Is TDS privacy for telegraph critically de¬ 
pendent upon rates of hand sending and lengths 
of TDS time elements ? 







24 


TIME DIVISION SCRAMBLING SYSTEMS 


3. What impairment to telegraph transmis¬ 
sion is caused by TDS ? 

4. Is it feasible to apply machine telegraph 
sending to TDS ? 

With regard to the relative privacy of tele¬ 
graph and speech, the data if taken literally 
indicate a slightly greater privacy for telegraph 
than for speech. The differences in the average 
solving times are so small as to be negligible, 
however. The solving times both for telegraph 
and speech vary from sample to sample, so that 
some telegraph samples were solved more 
quickly than some speech samples, but the oppo¬ 
site was likewise true, and the range of varia¬ 
tion was about the same for telegraph and 
speech. 

However, there is a difference of another 
nature which is inherent. The two main steps in 
the restoration of scrambled speech are, first, 
that the code be found and, second, that the 
scrambled speech be reproduced through TDS 
equipment arranged for the unscrambling code. 
These steps also can be used for unscrambling 
telegraph, and if they are used there is no dif¬ 
ference in the degree of privacy for speech and 
telegraph, since the times taken to ascertain the 
codes were the same. But another method might 
also be used for telegraph. If an oscillogram or a 
paper trace were made of the entire scram¬ 
bled message, then solution of the code would 
supply the formula according to which trace 
should be cut up and reassembled. If this were 
done for speech, the result would still be un¬ 
readable without fairly elaborate equipment; 
but a reassembled telegraph trace could be read 
by visual inspection of the dots and dashes. 

As a practical matter this difference appears 
to be of little importance, as the following cal¬ 
culation indicates: 

At 25 words per minute, a 100-word message 
takes 4 min, or 240 sec. At 0.75 sec per TDS 
cycle, this would cover % X 240 = 320 TDS 
cycles, or supply 20 X 320 = 6,400 TDS ele¬ 
ments for manipulation. If the trace were run 
so as to allow in. to each element, or 48 ele¬ 
ments to the foot, which would be extremely 


compressed, the trace would need to be 6400/48 
= 133 ft long. Even with practice and concen¬ 
tration probably at least a minute would be 
needed to reassemble each cycle, what with the 
manual labor of cutting and pasting, or 320 min 
in all; that is, it would take about 5 hr for one 
person to recover a 4-min message. Additional 
people or supplementary equipment would, of 
course, reduce this time. 

Thus, while the sense of the telegraph mes¬ 
sage can be recovered with less elaborate equip¬ 
ment than is needed for speech, the amount of 
clerical work involved would constitute a pro¬ 
tection lasting longer than the privacy time 
which should normally be associated with a 
fixed-code system. 

The answers to the questions posed above 
were found to be about as follows. 

1. Scrambled hand-sent telegraph signals re¬ 
quire at least as much time to decode as scram¬ 
bled speech; there is therefore no reason to be¬ 
lieve that the application of manual tone tele¬ 
graph to TDS will jeopardize its value for 
speech. 

2. The privacy of TDS for telegraph is not 
critically dependent on rates of hand sending 
and lengths of TDS time elements. 

3. The limits of telegraph transmission are 
reached with about 5 db less thermal (random) 
noise with the TDS than without; this means 
that the range is reduced unless the signal 
strength is increased. The impairment is less 
for short elements than for long TDS time ele¬ 
ments. 

4. Machine telegraph, both Boehme and tele¬ 
typewriter, give generally legible, though not 
letter-perfect, results with single-tone transmis¬ 
sion, if care is used. Better results, though still 
not perfect, may be obtained with two-tone 
transmission. 

The final report^^ of Project C-55 covers the 
individual points raised in the above questions 
in considerable detail and includes additional 
material, not summarized here, on the question 
of the quality of hand or machine sending as 
affected by TDS. 




Chapter 3 

SPEECH PRIVACY SYSTEM DEVELOPMENT 


3 1 RCA-BEDFORD SPEECH PRIVACY 
SYSTEM 

I N THE RCA-BEDFORD speech privacy system,'^ 
a portable system having short-term secu¬ 
rity, the speech wave is coded by multiplying 
it by an audio-frequency coding wave. A con¬ 
necting circuit between transmitter and re¬ 
ceiver having essentially faithful reproduction 
over a band of 100 to 4,000 cycles furnishes 
the required intelligibility and reliability. 


Basic Principles 


At the sending end a sound wave Sq is first 
‘‘compressed” to a uniform average amplitude 



Figure 1. Coding and decoding processes involved 
in RCA-Bedford multiplication privacy system. 


level by a special compandor circuit to form a 
signal S. The signal S is then multiplied by 
a suitable audio-frequency coding wave K to 
form a coded wave SK which is unintelligible. 
The expression “multiplied,” as herein used, 

a Project C-54, Contract No. OEMsr-592, Radio Cor¬ 
poration of America. 


means that the product wave SK has instan¬ 
taneous ordinates measured from its a-c axis 
which are proportional to the corresponding 
products of the original waves S and K. This 
is illustrated by waves S, K, and SK, Figure 1. 

At the receiving end the coded wave SK is 
multiplied by the “reciprocal” wave 1/K (which 
is derived from the locally generated code wave 
K) to produce the restored speech wave S', 
which is ideally like S. 

SKX-h= Sot S'. 

K 

As shown in Figure 1, the reciprocal wave 1/K 
has instantaneous ordinates which are propor¬ 
tional to the reciprocal of the corresponding 
coding K. 

The ordinates of the coded wave SK become 
zero each time the coding wave becomes zero. 
For these instants the reciprocal wave 1/K 
ideally should become infinite. Since this is 
impossible in practice, a narrow gap must occur 
in the restored wave S' as shown in the figure, 
at each time the wave K passes through zero. 
This amounts to an introduction of spurious 
signal components, which are largely removed 
by a low-pass filter. 

The decoded wave S', which is still com¬ 
pressed, is “expanded” to its original level by 
the receiving end of the compandor. The ex¬ 
panded signal So'y which is like Sq except for 
losses, is heard in the phones. 

The various treatments of the signal are in¬ 
dicated in the block diagram. Figure 2. 


Wave Multiplier 

The wave multiplier used to multiply audio¬ 
frequency waves Shy K and SK by 1/K is really 
a balanced modulator which is completely “bal¬ 
anced” in the sense that only the instantaneous 
product terms are produced. This balanced con¬ 
dition can also be described by saying that the 
output contains only the sideband frequencies 


25 















26 


SPEECH PRIVACY SYSTEM DEVELOPMENT 


of modulation; namely, those frequencies which 
are the sums or the differences of the frequen¬ 
cies of S and K. Ideally, no harmonics of either 
S or K are present in the output. This is an 


lator followed by filters to suppress the unde¬ 
sired frequencies. The present multiplication 
process could not be carried out in this manner 
because the frequencies it is desired to suppress 



RECEIVE 



Figure 2. Block diagram showing compandor in RCA-Bedford privacy system. 


important fact from the standpoint of security. 

This process would correspond to modulation 
as commonly used in radio transmission, except 
that in the privacy system, both the carrier and 
the speech waves are suppressed. In radio this 
can be accomplished in an unbalanced modu- 


occupy the same frequency range as the desired 
modulation sidebands. 

The special wave multiplier developed for the 
speech privacy system employs four small oxide 
rectifiers (known commercially as varistors) 
operating in their square law range. 























































RCA-BEDFORD SPEECH PRIVACY SYSTEM 


27 


Compandor 

The name '‘compandor’' is derived from the 
words “compress” and “expand.” It is an old 
device designed to improve the signal-to-noise 
ratio in communication by raising the level of 
transmission of the weaker sections of the 
speech signal with respect to the strong sections 
of the signal. This is equivalent to compressing 
the higher sections. At the receiving end the 
various sections of the signal are restored to 
their original relative levels. 

In the present case the compandor is used, 
primarily, to destroy the cadence (or loudness 
variation) of the coded signal, and thereby 
improve the security. To further this end, a 
special compandor circuit was developed, in 
which a pilot tone is used to fill the space be¬ 
tween words, and to provide control for the 
expanding process at the receiver. 

The curve So in Figure 3 represents the am¬ 
plitude, or envelope, of a word of speech. This 
is fed into the compressor (i.e., the sending part 
of the compandor) as shown in Figure 2. A 
filter removes any 1,000-cycle speech compo¬ 
nents, and then the output of a 1,000-cycle 
oscillator is added in the mixer. This gives the 
combined envelope shown as ''So + tone” in 
Figure 3. The amplitude of the tone is about 
10 per cent of that of the loudest section of the 
speech. The output of the mixer is detected, 
filtered to remove the audio frequencies, and 
then used to vary the bias of a pair of push-pull 
variable-mu tubes which amplify the mixed 
signal. This bias control is such as to make the 
combined signal S have a substantially constant 
loudness as shown at S in Figure 3. Note that 
the lower speech levels are raised. This signal 
contains a variable amplitude of 1,000-cycle 
tone, as shown at T. The signal S is then coded 
by multiplying by the code wave K, and trans¬ 
mitted. 

At the receiver, as shown in Figure 2, the 
received signal SK is decoded to S'. In the ex¬ 
pander, the 1,000-cycle tone is selected by a 
filter, and then detected to control the gain of 
a pair of variable-mu tubes. The variation in 
gain is such as to restore the amplitude of the 
tone component of the output to a constant 
value, and simultaneously restore the speech 


level to its original proportions. A filter then 
removes the control tone, and a 3,000-cycle low- 
pass filter removes the high-frequency distor¬ 
tion components from the phone circuit. 





Figure 3. Envelopes of speech and tone in 
compandor. 

When the signal transmitted in this manner 
is received on a privacy unit, operating with 
a code wave that differs substantially from the 
code wave at the sending end, the pilot (1,000- 
cycle) tone is spread irregularly over the audio 
spectrum so that a filter cannot isolate it for 
operating the expander to restore cadence. 
Neither can a filter be used to remove it from 
the phone circuit. The result is an unintelligible 
mass of coded speech and noise. Tests show that 
to get intelligibility from the decoded signal, 
the sending code wave and the receiving code 
wave must have a higher degree of similarity 
than when the compandor is not used. 

Inspection of Figure 2 shows that most of 
the parts of the privacy unit, used in sending, 
are the same as used in receiving. Therefore, in 
a send-receive unit, the use of suitable switches 
or a relay avoids duplicating these parts. 











28 


SPEECH PRIVACY SYSTEM DEVELOPMENT 


Generating Code Wave by Delay 
Network 

The coding wave K in the system is generated 
by combining with different changeable polari¬ 
ties the outputs from taps along a delay network 
which is fed with narrow pulses from a fixed- 
frequency multivibrator, illustrated in Figure 4. 
The network contains 80 sections of series in¬ 
ductance and shunt capacitance. A repeater and 
an equalizer (not shown in the figure) are in¬ 
serted at 16-section intervals to make up for 
attenuation along the network. 



JV 


0-4 ---— 






-yi/l/lAA/VVvy^- 

A' * 





Figure 4. Generation of coding wave K. 


The multivibrator [MV] supplies repeating 
pulses to the input A of the delay network, as 
shown by wave A in the figure. The frequency 
of wave A is such that immediately after a 
pulse has reached the far end of the network, 
another pulse is fed into the near end. Then, 
neglecting distortion for the present, the volt¬ 
ages at tap points B, C, D, etc., are like A ex¬ 
cept delayed various amounts in time, as shown. 

Each tap is shown here with a SPDT switch 


so that it can be connected through suitable 
buffer resistors to point P, either directly or 
through a polarity-reversing amplifier [RA]. 
(Actually, the switching arrangements are 
more complicated than shown here, as will be 
explained, but Figure 4 suffices for an ele¬ 
mentary explanation.) The voltage at point P, 
therefore, consists of the sums of the various 
voltages along the network taken either posi¬ 
tively or negatively, depending upon the switch 
positions. Wave H illustrates various pulses 
which would combine to form a wave such as /, 
which has a variety of widths of lobes. From 
this simple illustration, it is clear that a variety 
of complex waves, having a repetition rate of 
100 times per second, could be produced by 
various switch settings. 

Actually, the multivibrator tends to produce 
a very narrow repeating pulse, as shown at 
instead of a smooth, wide pulse, as shown at A. 
However, the multivibrator shock excites the 
first section of the network, resulting in an 
oscillatory voltage wave, as shown at L, to 
occur at tap A. As the pulse progresses along 
the network, the phase distortion of the net¬ 
work (which is very pronounced shortly below 
cutoff frequency) causes the pulse to have an 
extended oscillatory “tail” as shown by wave M. 
The voltage at point P then is the sum of many 
waves which are, themselves, quite complex in 
shape, and may have lobes with a large variety 
of amplitude, as well as widths as shown at K 
in the figure. 

In this privacy system, the instantaneous 
voltage of the transmitted signal SK is propor¬ 
tional to the instantaneous value of wave K. 
Then, if the coding wave K were allowed to 
have peak amplitudes far in excess of its aver¬ 
age amplitude, the average utilization of the 
available transmitter power would be very low. 

Accordingly, to improve the power efficiency 
of speech transmission, the high peaks of wave 
K of Figure 4 are limited to amplitudes consid¬ 
erably lower than the original amplitudes, 
shown by the dotted peaks. The limiting action 
is gradual in order to retain some amplitude 
variation in the code wave. After the limiter, 
a small shunt capacitor serves to smooth over 
the discontinuities in the wave caused by lim¬ 
iting. 






































RCA-BEDFORD SPEECH PRIVACY SYSTEM 


29 


3.1.5 5ynehronization of the Delay Network 

Consideration of the decoding process shows 
that it is necessary for the receiving code wave 
K to have the same timing as the code wave at 
the sending apparatus (after making suitable 
allowances for the time of transmission of the 
signal). To maintain this condition, a syn¬ 
chronizing pulse is transmitted along with the 
signal SK, to control the multivibrator pulsing 
the network of Figure 4 when receiving. 

Briefly the coded speech signal is inter¬ 
rupted for about 0.001 sec each 0.01 sec and a 
synchronizing pulse is inserted. At the receiver 
this pulse is selected by virtue of the regularity 
of its occurrence (as compared to any lobe of 
the coded speech) and used to trigger the re¬ 
ceiving multivibrator which pulses the delay 
network which, in turn, generates the code 
wave. 

^ ^ ^ Code-Changing Method 

The eight code disks which are part of a 
rotary code-changing switch may be seen in the 



Figure 5. Front view of Model RCAL-1 privacy 
equipment. 


photograph of Figure 5. Each code disk has 
twenty insulated silver segments, of three dif¬ 
ferent types, which are located in irregular 


order in a circle. A brush holder with five sets 
of three brushes is located adjacent to each 
code disk so that each set of brushes acts as a 
SPDT switch which is open, positive, or nega¬ 
tive depending upon which silver segment is 
under the brush. Each of these SPDT switches 
controls two taps on the delay network through 
two buffer resistors. The order of connections 
between the 80 delay network taps and the 
forty switches is irregular. 

The eight code disks are adjustably mounted 
on a drum by eight individual detents so that 
each disk can be set manually to any one of 
twenty possible positions. The shaft supporting 
the disks is rotatable by a ratchet-operated 
solenoid in steps of 1/100 revolution. The sol¬ 
enoid is energized at 3-sec intervals by a contact 
controlled by a spring-driven clock. A time disk 
fixed to the right-hand end of the drum is cali¬ 
brated in 100 equal divisions. 

® Auxiliary Compandor and Equalizer 
Unit 

In the coding process used in Model RCAL-1, 
each original speech frequency component is 
shifted both upward and downward in fre¬ 
quency by each frequency component of the 
code wave. This method diffuses the frequency 
components of the speech quite thoroughly. 
Therefore, it is believed that most of the intel¬ 
ligibility obtained by listening to the coded 
wave SK is due to the cadence of the words. To 
destroy the cadence of the speech to be scram¬ 
bled and therefore to improve the security, a 
special compandor described briefly in the be¬ 
ginning of this summary was developed. 

3.1.8 Weakness of Short Repeating 
Code Waves 

In Figure 6, a code wave K is shown multi¬ 
plied by a section of speech wave Sa to form 
the coded wave S^K. The coded wave crosses the 
axis each time the code wave crosses its axis, 
and also when crosses its axis. Similarly, a 
different section of speech wave Sj^, when mul¬ 
tiplied by the same section of code wave K, 
results in coded wave S^K. In this wave, those 
crossovers produced by K correspond in time 






30 


SPEECH PRIVACY SYSTEM DEVELOPMENT 


with crossovers in wave S^K, but other cross¬ 
overs do not agree. 

At SK are superimposed several sections of 
speech wave coded by identical sections of code 
wave, such as would occur with the repeating 
code wave of Model RCAL-1. This is substan¬ 
tially the image seen in an oscilloscope picture 
of the coded signal when properly synchronized 
by the transmitted synchronizing pulses. In¬ 
spection of this image readily reveals the prob- 



Figure 6. Wave forms demonstrating weakness 
of short repeating code wave. 

able location of the crossovers of the code wave, 
and also indicates the shape of the coding wave. 
This is particularly true if a large number of 
waves are superimposed, as seen on an oscillo¬ 
scope with a long-retentivity screen. It has 
been demonstrated that a rectangular wave, as 
shown at K', can be used effectively to decode 
a message transmitted by the code wave K. 
Hence, it is only necessary to determine, and 
make suitable use of, the location of the cross¬ 
overs of the code wave to crack the message. 

The code wave of Model RCAL-1 has (by 


count), on the average, about 25 crossovers 
which repeat at intervals of 0.01 sec. The cross¬ 
overs are spaced at irregular intervals, depend¬ 
ing upon the settings of the code switches. The 
wave is composed of the various harmonics of 
the 100-cycle fundamental from about 300 to 
2,500 cycles per second. The harmonics have 
various phases and amplitudes. This code wave 
is changed, in part, each II /2 sec through a 
5-min cycle by the clock-driven rotary switch 
which contains the eight code disks mentioned 
above. Ten of these small partial changes, oc¬ 
curring in 15 sec, may cause a complete change 
of all contacts. Therefore, only sections of about 
5 sec of the message would be cracked by a 
single code wave. 

Although considerable apparatus of a very 
special character would be needed to actually 
crack a message coded with such a repeating 
code wave by making use of the repeating pat¬ 
tern of crossovers in the coded wave, as deter¬ 
mined by statistical observation, nevertheless 
it must be appreciated that a suitable accurately 
calibrated oscilloscope could be used to observe 
the coded wave and determine the timing of the 
crossovers of the code wave, and that a suitable 
special apparatus could then be adjusted to 
produce a decoding wave such as K' of Figure 6 
to crack the message. Furthermore, there is a 
feasible plan for an apparatus to do the entire 
job automatically. Hence, no coding wave which 
repeats in a reasonable time can provide a high 
degree of security against an able enemy. 

For the sake of completeness of this discus¬ 
sion, it should be pointed out that a message 
coded by a code wave such as K, in Figure 6, 
can be decoded by using a receiving code wave, 
such as A", which differs considerably from K. 
From observation, it is estimated that if most 
of the crossovers of the receiving code wave 
occur within 20 or 30 per cent of the time of 
occurrence of the crossovers of the sending code 
wave, the message can be cracked even when 
the compandor is used. (The per cent value used 
here is based upon the intervals between ad¬ 
jacent crossovers in the sending code wave 
being 100 per cent.) Also, a few extra cross¬ 
overs in either code wave, having no corre¬ 
sponding crossovers in the other wave, may be 
tolerated. With such a wide tolerance in the 






























RCA-BEDFORD SPEECH PRIVACY SYSTEM 


31 


cracking code wave, it is not surprising that in 
Model RCAL-1, which uses a repeating code 
wave having an average of only 25 crossovers, 
a short message can be cracked in a short time 
by “playing'' with the code disks, or switches, 
at random. 

Since all limitations of security of Model 
RCAL-1 with the compandor arise directly, or 
indirectly, from the repetitive character of the 
coding wave, the use of a nonrepeating code 
wave would appear desirable. 


Dual Delay Networks for Generating 
Long Code Wave 

In a system providing longer security, the 
period of the code wave could be made 0.4 sec 
(as compared to 0.01 sec) by using two delay 
networks operating at slightly different pulse 
frequencies to generate the code wave. This is a 
purely electrical device. Then the mechanical 
apparatus for changing taps on the networks 
can operate at the relatively low speed which 
causes a substantially complete change in each 
0.4-sec interval, and thereby prevent the code 
wave from repeating itself until the mechanical 
system has run through its complete cycle. The 
time for this is over an hour. 

The dual delay network appears to be a very 
economical electrical device for generating a 
code wave with a period as long as 0.4 sec. To 
generate a wave of this period and maximum 
frequency directly by a single delay network, 
would require about 3,000 LC sections. 


3.1.10 Motor-Driven Tap Switch with 
One-Hour Period 

In the dual delay system, it is required that a 
critical number of taps on the dual delay net¬ 
works be changed each 0.4 sec to prevent a repe¬ 
tition of the code wave 2.5 times a second, which 
would allow a statistical study of the coded 
wave SK to reveal information as to the loca¬ 
tion of crossovers in the code wave K. This is 
accomplished by a three-speed motor-driven 
rotary switch. 

The switch, designed for the proposed long- 


security system has nine code disks and brush 
holders instead of the eight in Model RCAL-1. 
The 45 SPDT switches provided are connected 
in irregular order to control the 84 taps in the 
dual delay networks. These nine code disks are 
rotatably mounted with individual detents in 
groups of three on three separate drums. The 
three drums are geared to run at slightly differ¬ 
ent speeds, which are, respectively: 

>Si = 6 rpm, 

20 

1 S 2 = 6 X ^ = 5.71 rpm, 

20 

aSs = 6 X Yg = 6.31 rpm. 

To the left of each group of code disks is a 
timing disk which is fixed to the drum so that it 
controls all three code disks in its group. Each 
of three 40-tooth gears is rotatably mounted on 
a counter shaft, but is restrained from relative 
rotation by an eight-position detent. These 
three detents allow the three timing disks to be 
independently adjusted to their proper starting 
positions by hand. The timing disks are cali¬ 
brated, and numbered in 20, 21, and 19 equal 
divisions, respectively, to agree with the detent 
positions on the counter shaft acting through 
the three different gear ratios. All code disks 
have twenty positions on the drums, indicated 
by letters. 

The speeds of the three groups of code disks 
are such that the system requires about 1 hr 
6 min to run through a complete cycle, so that 
all groups reach their starting position simul¬ 
taneously. Therefore, by resetting the individ¬ 
ual code disks to a new code each hour, a non¬ 
repeating code wave is provided. Any two 
groups of the disks go through a cycle in about 
3 min. Then, if the effect of the other group of 
three disks upon the code wave were suffi¬ 
ciently small, the code wave would effectively 
repeat about twenty times each hour, and be¬ 
come accessible to the enemy. 

Tests, however, made with Model RCAL-1, 
operating with the compandor and with a tem¬ 
porary motor-drive for the disks, showed that 
with three of the eight disks off code only 6 per 
cent of words were understood. (Each word was 
repeated three times.) With two of eight disks 
“off," 25 per cent were correctly understood. 




32 


SPEECH PRIVACY SYSTEM DEVELOPMENT 


To keep the motor in the receiving apparatus 
in synchronization with the motor at the send¬ 
ing station, both synchronous motors are driven 
by 60-cycle power whose frequency is controlled 
by a 145-kc crystal. 


^ ^ Probable Security of the System 

Based on tests made on the Model RCAL-1 
with compandor and a temporary motor-drive, 
it is concluded that, to obtain from a captured 
machine adequate intelligibility to crack a mes¬ 
sage, the enemy would have to have at least six 
of the nine code disks in the correct position. 
The probability of this condition being obtained 
in a single trial, at random, is 1/889,000. 

It is estimated that at least 3 sec would be 
needed to set up and try each of the many pos¬ 
sible code disk positions, even if elaborate spe¬ 
cial means were built to allow orderly adjust¬ 
ment of the relative positions of the disks while 
running. (The time for trying each code setting 
is made longer because of having to make a 
short exploration for correct phase of the entire 
disk assembly.) Hence, the average time for 
reaching the condition for six code disks right 
is: 


Time = 


3 X 889,000 
2 X 60 X 60 


= 370 hr. 


(The use of more cracking setups would shorten 
this time.) 

If this time is deemed inadequate to provide 
the desired security, it could be greatly ex¬ 
tended by adding a few more code disks. An 
alternative method would use a selector switch 
of the radio-band-selector type for manually 
changing the connections between the motor- 
driven switch and the taps on the delay net¬ 
work. The setting of this switch would be part 
of the code. 

No reasonable method of cracking messages 
sent by this system, other than the trial method 
above, occurred to the RCA engineers. Bell 
Laboratories’ engineers (Project C-43) and 
others were given complete verbal descriptions 
of the proposed system, and invited to find some 
property of the coded wave by which it might be 
decoded. No likely property was revealed.^^ Of 


course, it cannot be known with certainty that 
the signal could not be cracked eventually by a 
shorter method. 

By contrast, the basic weakness of the Model 
RCAL-1, with its repeating code wave, was 
known and reported very early, that is, well 
before design was begun. The clock-driven code 
switch was included with the expectation that 
it would only make cracking a more laborious 
process. (After the model was built, it was 
found that a message could be cracked by “play¬ 
ing” with the code disks somewhat quicker than 
originally expected. This was because it had not 
been appreciated how different the receiving 
code wave could be from the sending code wave 
and still give appreciable intelligibility.) 

In studying the probable security of the pro¬ 
posed system, it should be remembered that the 
signal S (which is coded to form SK) usually 
contains a very substantial amount of the 1,000- 
cycle compandor control tone. Offhand, this 
fact would seem to offer the most likely ap¬ 
proach to a reasonable method of cracking. If 
this were found to be so, it would be necessary 
to use some other apparatus to replace the com¬ 
pandor. Two different devices are available but 
they are not attractive. 


3 . 1.12 Pj.gg 0 j^j Status of the Development 

Part I of the final report^^ of the project gives 
an assessment of the speech quality transmitted 
by the system and indicates that radio trans¬ 
mitters and receivers somewhat better than 
actually used during the war would be required 
to deliver good intelligibility. Weights and di¬ 
mensions are to be found in the final report. 

At the close of the project most of the new 
circuits required in the proposed high-security 
system had been developed. Some of these de¬ 
veloped and used in Model RCAL-1 are: the 
wave multiplier, the reciprocal circuit, delay 
network and tap switching circuits, synchroniz¬ 
ing circuits, synchronizing blanking circuits, 
and the compandor. 

The following additional circuits were devel¬ 
oped, or tested specifically to be used in the 
proposed high-security units: the “clip, multi¬ 
ply, and phase-distort” method for blending the 





HAZELTINE BAND DISPLACEMENT SYSTEM 


33 


outputs of two delay networks was tested and 
shown to produce a coding wave with more 
crossovers and with the initial crossovers ade¬ 
quately ‘'smeared/' A synchronous motor suit¬ 
able for driving the code disks had been driven 
by an amplifier from a chain of frequency- 
dividing multivibrators. 

The rotary code switch had been designed, 
but not detailed. It included code disks, brushes, 
brush holders, and detents, which are very simi¬ 
lar to parts in Model RCAL-1. 

Conclusion 

It was concluded that the speech privacy unit 
proposed would probably have a relatively high 
degree of security, that it would have dimen¬ 
sions and power requirements which allow it to 
be portable, and that it could be developed with 
reasonable expenditure of time and money. 

3 2 HAZELTINE BAND DISPLACEMENT 
SYSTEM 

The Hazeltine speech secrecy system^ is of 
the band displacement type in which the speech 
is inverted and displaced in frequency to seven 
successive positions in an extended a-f band, in 
a sequence determined by a coding switch and 
punched cards. 

^ ^ ^ General Description of the System 

In transmitting, speech is first passed 
through a frequency selective network in which 
the frequency spectrum is tipped up at the rate 
of 6 db per octave. It is then modulated with a 
13-kc carrier and the upper sideband extending 
from 13 to 16 kc selected in a band-pass filter. 
This is again modulated by a selected sequence 
of seven displacement carriers, 1 kc apart from 
17 to 23 kc. The lower sideband is then selected, 
resulting in inverted speech in a sequence of 
bands in a range from 1-4 to 7-10 kc. Amplifiers 
are employed in the carrier channel to maintain 
the output at the same level as the input. 

In receiving, the same apparatus is used. The 
speech is passed through the system in the re- 

b Project C-15, Contract No. NDCrc-139, Hazeltine 
Corporation. 


verse direction for unscrambling and at the 
normal speech end of the channel the frequency 
spectrum is tipped down by 6 db per octave to 
restore the original frequency characteristic. 

An electronic switch for keying on the 17- 
to 23-kc displacement carriers for 50-msec in¬ 
tervals in a selected sequence comprises a chain 
of seven switching tubes. Each of these is ar¬ 
ranged to generate a 50-msec pulse. The termi¬ 
nation of this pulse initiates operation of the 
succeeding switching tube. In transmitting, the 
pulse from the last of the switching tubes is 
returned to the first to make the operation con¬ 
tinuous. The pulses derived from the switching 
tubes are delivered to the seven carrier fre¬ 
quency oscillators as selected by the coding 
switch and serve to key-on the oscillators. 

To synchronize the receiver and the trans¬ 
mitter, the pulse derived from the last of the 
switching tubes in transmitting is suitably am¬ 
plified, limited, and filtered to eliminate com¬ 
ponents above 1 kc and then added to the scram¬ 
bled speech at the end of the carrier channel. 
In receiving, this synchronizing pulse is sepa¬ 
rated from the scrambled speech and, after am¬ 
plification and limiting, is applied to the first of 
the switching tubes. The connection between 
the last switching tube and the first is elimi¬ 
nated in the receiver so that operation of the 
electronic switch in receiving is initiated by re¬ 
ceived synchronizing pulses only. 

To reduce the possibility of actuation of the 
electronic switch by spurious pulses, a barrier 
circuit in the form of a pulse generator having 
slightly less than a 350-msec period, is placed 
ahead of the first switching tube. Once a syn¬ 
chronizing pulse actuates the barrier circuit 
(and simultaneously the first switching tube) 
the barrier circuit becomes immune to further 
pulses during its cycle. This prevents any pulses 
from reaching the first switching tube until al¬ 
most the entire cycle of the seven switching 
tubes has been completed. 

3 .2.2 Performance of First Two Models 

Two models embodying such a system were 
completed, tested, and demonstrated to mem¬ 
bers of Division 13. Although it was possible to 
obtain speech of reasonably understandable 





34 


SPEECH PRIVACY SYSTEM DEVELOPMENT 


quality after a complete cycle of scrambling 
and unscrambling, it was felt that the perform¬ 
ance could be appreciably improved in several 
ways. 

It was felt desirable to reduce the degrada¬ 
tion of speech quality obtained with the equip¬ 
ment. This degradation was probably caused by 
the following factors. The oscillators did not 
respond immediately to the keying pulses so 
that intervals of several milliseconds were left 
at the transition points for the displacement 
carriers. This resulted in blanks in the speech 
and in transient clicks. The frequency stability 
of the oscillators was poor with the result that 
synchronization between oscillators in the 
transmitter and those in the receiver could not 
be held. This resulted in slight changes in pitch 
of the speech as the displacement carrier fre¬ 
quency changed. There was also some nonline¬ 
arity present in some of the amplifiers. 

It was also found that when as little as 10 db 
attenuation was inserted in the line between 
the transmitter and the receiver, the receiver 
had a tendency to drop out of synchronization 
with the transmitter. It was felt that the equip¬ 
ment should be designed to tolerate an attenu¬ 
ation of at least 25 db below the normal level. 
No experience had been obtained on the effect 
of noise and code interference on the synchro¬ 
nization of the two units. It was felt that rea¬ 
sonable immunity to such interference was an 
important requirement of the equipment. 


Construction of Third Model 

With the experience gained in the operation 
of the first two models, the construction of a 
third model was undertaken. The objectives 
sought in this third model were as follows: 

1. Improved oscillator frequency stability. 

2. Fast keying on and off of oscillator. 

3. Elimination of nonlinearity in amplifiers 
in the carrier channel. 


4. Substantially flat overall frequency char¬ 
acteristic from 300 to 3,000 cycles per second. 

5. Improved immunity to changes of level 
and noise in the synchronization of the receiver. 

6. Incorporation of a storage battery oper¬ 
ated power supply. 

7. Improved isolation and shielding of com¬ 
ponents to avoid carrier leakages and cross 
modulation. 

8. Incorporation of a code switch suitable for 
operation with a punched card. 

9. Closer approximation to a production de¬ 
sign. 

The third model, completed in the latter part 
of September 1941, met the above objectives 
very well. Speech quality was materially im¬ 
proved over that obtained with earlier models. 
Switching transients between successive dis¬ 
placement carriers could not be detected in the 
output. The prominence of the sync pulse in the 
output was greatly reduced. Measurements in¬ 
dicated that the distortion of the system was 
reasonably low. The input to the receiver could 
be reduced more than 25 db below normal level 
before the receiver dropped out of synchronism. 
With high noise levels the receiver remained in 
synchronism at least up to the point where the 
noise was equal to the signal. 


Apparatus Details 

The final report^^ on the project gives details 
of the transmitting and receiving circuits, 
means for providing nonscrambled speech when 
desired, method of setting up the desired code 
by means of punched code cards, an analysis 
of the inherent secrecy of the system plus the 
chances of cracking it, and suggested improve¬ 
ments. 

The equipment weighed 45 lb, required ap¬ 
proximately 1.14 cu ft of space, operated from 
6.3 volts and needed 10 amp at this voltage. 




Chapter 4 


UNSCRAMBLING AND DECODING METHODS 


4.1 HISTORY 

A long with projects and work primarily 
^ concerned with the development of scram¬ 
bling methods for speech, code telegraph, or 
facsimile, projects concerned with unscram¬ 
bling or code-cracking methods engaged a large 
part of the time and energy of those associated 
with this portion of Division 13 activity. In this 
manner the effectiveness of the privacy methods 
developed could be tested constantly. 

Realizing early in the privacy research con¬ 
ducted under the sponsorship of Division 13 
that the ear had very limited capabilities for 
analyzing scrambled speech, the sound spectro¬ 
graph was developed by the Bell Telephone 
Laboratories to provide speech patterns which 
could be interpreted by the eye. In effect this 
valuable instrument divided scrambled speech 
into its three important dimensions of fre¬ 
quency, amplitude, and time. By its means, any 
alterations to the original speech in either fre¬ 
quency or time could be detected and analyzed 
with the object of adjusting unscrambling ap¬ 
paratus so that the original speech could be 
recovered. 

Early in 1941 a rough laboratory model of 
the sound spectrograph was demonstrated to 
the National Defense Research Committee 
[NDRC] and as a result Project C-32 was or¬ 
ganized with the immediate object of producing 
such a device in a form that would be useful 
for diagnosing and decoding speech scrambling 
systems. Such a model was produced and suc¬ 
cessfully demonstrated to representatives of 
NDRC, Army, and Navy. 

Upon the termination of Project C-32 on 
February 1, 1942, it was decided that the work 
initiated under that project should be continued. 
Accordingly Project C-43, “Continuation of 
Decoding Speech Codes,” was authorized. The 
project anticipated some routine decoding, the 
production of duplicate equipment to be used 
by the Army and Navy intelligence services, 
and further studies of decoding tools and meth¬ 
ods. At that time the Army and Navy were 


relying almost entirely upon this project to 
furnish the above services until they could be 
provided with suitable equipment and could 
obtain trained personnel. Based on the needs 
of the military, this project was thrice ex¬ 
tended. 

Under the guidance of NDRC Division 13, 
the emphasis was placed at any given time on 
what was deemed to be most urgent. This is 
reflected in the subject matter of the prelim¬ 
inary reports of Project C-43 which were issued 
from time to time and which form the appendix 
to the final report of that project. In addition 
to the specific investigations covered by these 
preliminary reports much work was carried on 
as the basis for more general coverage of the 
field of interception, diagnosis, decoding, and 
evaluation of speech privacy systems. 

In addition to the general studies mentioned 
above, decoding equipment was developed and 
models furnished to the Army and Navy. This 
decoding equipment included (1) two models of 
the sound spectograph, (2) a variable-area pat¬ 
tern machine, and (3) equipment for decoding 
two new enemy privacy systems intercepted by 
the project personnel at Point Reyes, California. 
In each case Army and Navy personnel were 
instructed in the operation and maintenance of 
these equipments. 

Intercept activities of the Project C-43 per¬ 
sonnel included (1) the study of recordings sub¬ 
mitted early in the project by the Federal Com¬ 
munications Commission, (2) exploratory work 
at the Bell Telephone Laboratories experimental 
radio receiving station at Holmdel, New Jersey, 
and (3) exploratory work and routine intercep¬ 
tion of radio telephone transmissions at the 
American Telephone and Telegraph Co. radio 
receiving station at Point Reyes, California. Re¬ 
ports of the results of the above studies and 
recordings of intercepted material were sub¬ 
mitted directly to the interested military au¬ 
thorities. 

Many speech privacy schemes were submitted 
through NDRC during the course of this proj¬ 
ect. These were studied and evaluated. This 


35 


36 


UNSCRAMBLING AND DECODING METHODS 


work led directly to the continued improvements 
of the sound spectrograph and the development 
of supplementary decoding tools and techniques. 

As the Army and Navy became able to carry 
on decoding activities themselves with the aid 
of equipment and information furnished by 
NDRC as the result of work outlined above, the 
activity on Project C-43 gradually decreased. 
The final reports on the several projects cover 
all phases of the work on the general subject 
and constitute a reference work for future 
studies of speech privacy systems. 

In this chapter will be found, first, some gen¬ 
eral observations on the intercept problem and 
on methods of cracking scrambled speech, a 
general description of the sound spectrograph 
and examples of its applications, and some ma¬ 
terial on the practical evaluation of privacy 
systems, all taken from the final report on 
Project C-43.^ Then follows a summary of the 
work accomplished in the several decoding proj¬ 
ects working under Division 13 sponsorship. 


INTERCEPTION 

Speech privacy systems may be used in con¬ 
nection with radio telephone systems or with 
wire systems. The unauthorized interception of 
wire communications in wartime, however, was 
beyond the scope of the work done for Division 
13. These notes therefore are confined to radio 
interception problems and expands the material 
in Preliminary Report No. 25.^^ The decoding 
techniques to be described subsequently, of 
course, apply to wire as well as radio communi¬ 
cations. 

Types of Radio Systems 

Radio telephone systems range in size and 
complexity from high-power point-to-point sta¬ 
tions operating over great distances to the low- 
power, short-range sets carried by individual 
soldiers. The high-power systems are usually 
designed to operate between specific points, 
using specific assigned frequencies. They are 
equipped with elaborate fixed antennas, which 
are usually of the directive type. Privacy equip¬ 
ment associated with such terminals may be as 
large and complex as desired to achieve virtual 


secrecy. A major consideration in such systems, 
of course, which adds to size and complexity, 
is that the privacy must not degrade the quality 
of the received speech to any appreciable extent. 

On the other hand, anyone can intercept these 
high-power signals at great distances, where he 
can have a well-equipped centralized decoding 
laboratory, with no limitation on the size and 
complexity of the decoding equipment he might 
bring to bear. This laboratory can be adequately 
manned by a relatively few highly trained de¬ 
coding specialists not necessarily members of 
the Armed Services. 

In contrast with this situation, the low-power, 
short-range radio sets used in military opera¬ 
tions are severely restricted as to size and 
weight, and these restrictions also apply to 
privacy equipment. The smallest privacy set 
submitted to Project C-43 for study was roughly 
a 10-in. cube, and was designed for mobile ap¬ 
plications like tanks, planes, and command cars. 
While it is difficult to achieve a high degree of 
inherent privacy in mobile equipment, it should 
be noted that the very mobility of such systems 
adds to the security, because the signals can 
not generally be picked up at great distances, 
and whatever equipment an intercepter might 
use to crack the privacy must also be mobile. 
Furthermore, the decoding equipment must be 
operated by military personnel, a large number 
of whom may be required if the enemy is making 
extensive use of mobile privacy. 

Intermediate types of radio systems are used 
for the higher echelons of command. For such 
applications, the radio equipment is semimobile. 
It can be transported in trucks and set up very 
rapidly, and may have a considerable range. 
For such applications, a high degree of privacy 
is required, and a truckload of equipment might 
be justified, because the enemy could afford to 
devote considerable time, personnel, and equip¬ 
ment to decoding the kind of messages which 
would be transmitted over such systems. 

Intercepted Signal Quality 

Since most of this chapter deals with decod¬ 
ing, the material from this point on will be 
written from the point of view of the unau¬ 
thorized rather than the authorized listener. It 
is first of all desirable to get a good signal, as 




INTERCEPTION 


37 


free as possible from interference. There are 
several reasons for this. First, the process 
which unscrambles the speech also scrambles 
any noise such as static which has been super¬ 
posed on the scrambled signal. This changes the 
time or frequency distribution of the noise, 
breaks up harmonic relationships, etc., thereby 
increasing the interfering effect of the noise. 
Second, the decoding is apt to be less perfectly 
accomplished than at the authorized terminals, 
which tends to make the speech harder to un¬ 
derstand. Finally, there are usually language 
differences which still further add to the diffi¬ 
culty of understanding the message. Conversa¬ 
tions can be carried on under extremely un¬ 
favorable conditions by people speaking their 
own language, but noise and poor quality rap¬ 
idly degrade the intelligibility of a language 
foreign to the listener. 

In this connection it might be noted that it is 
very desirable to be able to hear both sides of 
the conversation without interruptions, in order 
to follow the context. In the case of the point- 
to-point systems, this will in general require 
two receivers because the two directions are 
transmitted over separate channels at different 
frequencies. If the two outputs are mixed for 
listening or recording, however, it should be 
kept in mind that the noise on the weaker signal 
will be superposed on the stronger signal and 
may seriously degrade it. Putting the two sig¬ 
nals on two headphones will improve this situa¬ 
tion, because noise in one ear does not seriously 
affect the intelligibility of a signal in the other 
ear. This problem does not arise in the case of 
the smaller radio systems, because these are 
generally operated on the basis of switching 
between transmitting and receiving conditions 
on the same carrier frequency. 

Methods of obtaining a good signal are the 
same for the interceptor as for the intended re¬ 
ceiver. A few of the important considerations 
are listed here; further information on any or 
all of them can be had from radio reference 
works. (1) Point-to-point systems usually em¬ 
ploy directive antennas; the intercept station 
should therefore be located along or near the 
line of the radio beam. (2) In locating stations 
to intercept radio transmissions in the h-f range, 
account should be taken of the skip distances 


of the frequencies involved. Better signals will 
sometimes be obtained by moving farther away 
from the transmitter rather than closer. (3) 
The use of directive antennas, directed towards 
the transmitter being monitored, will improve 
the signal-to-noise ratio by discriminating 
against noise which is nondirectional. These 
antennas of course should be designed for the 
frequency and polarization of the signal, and 
properly coupled to the receiving set. (4) 
Stronger radio signals will be received if the 
antennas are located in the open, with no trees 
or other obstructions in the foreground. This is 
particularly important in the v-h-f range. (5) 
Radio signals increase in intensity as the height 
of the antenna above the immediate foreground 
is increased, particularly for v-h-f transmission. 
Thus better results are obtained with the an¬ 
tennas located on high masts or on hills over¬ 
looking the foreground in the direction from 
which the signal is arriving. If the signal is in 
the v-h-f range and other measures are inade¬ 
quate, it may even be desirable to consider 
receiving the signal in an airplane and record¬ 
ing it or retransmitting it for decoding. (6) 
Noise improvement can generally be obtained 
by keeping the receiving equipment away from 
sources of man-made noise, such as ignition 
systems and power lines. 

Receiving Sets 

With regard to the receiving sets, a distinc¬ 
tion must be made between the various ac¬ 
tivities of an intercept station. One important 
activity is searching for possible enemy trans¬ 
mission channels. The object is to determine all 
the channels in use, the location of their ter¬ 
minals, the type of business transacted, and, 
most of all, whether any special form of pri¬ 
vacy is used on the channel. Some preliminary 
searches of this type are described in Pre¬ 
liminary Reports No. 2 ^^ and 23.^^ If no privacy 
is used, other than the usual commercial types, 
it is unlikely that information of military im¬ 
portance is transmitted over the channel, and 
it may not be necessary to monitor it continu¬ 
ously. If a new privacy system is located, how¬ 
ever, it is very likely to be worth monitoring 
and decoding continuously. 

For the searching and scanning activities, the 



38 


UNSCRAMBLING AND DECODING METHODS 


ordinary commercial sets of the '‘communica¬ 
tions'' type, equipped with a beat-frequency 
oscillator, will serve very well for all types of 
transmission. Even the suppressed carrier type 
can be handled very well provided the signal 
is fairly strong. It may require continual 
manual adjustment of the local oscillator, but 
sufficiently good reception can be obtained to 
determine the nature of the channel. Cases of 
extreme spread-band transmission can also be 
handled in this manner. 

If a particular channel employing suppressed 
carrier is determined to be worth monitoring 
continuously, then a single-sideband receiver 
will give improved reception. These receivers 
are equipped to amplify the partly suppressed 
carrier, or supply a new one with great stability, 
and they may provide as much as 15-db improve¬ 
ment in signal-to-noise ratio in some cases. 
They also permit selecting either the upper or 
the lower sideband of double-sideband systems, 
which may be of advantage in cases where in¬ 
terference occurs on one or the other sideband 
of such systems. However, these receivers are 
not suitable for searching. 

Types of Radio Transmission 

A knowledge of the types of radio transmis¬ 
sion which may be encountered is very im¬ 
portant to the personnel of an intercept station. 
Experience has shown that without such knowl¬ 
edge, the nature of intercepted signals may be 
completely misinterpreted. It is possible to mis¬ 
take certain normal types of transmission for 
new systems, or conversely to fail to recognize 
new systems which should be monitored at once. 

Double-Sideband. The commonest type of 
transmission is the double-sideband type in 
which the carrier is transmitted along with the 
sidebands, which are usually about 3 kc in 
width, and are located immediately adjacent to 
the carrier. These are readily demodulated by 
the ordinary receiver. This is true even if the 
carrier is rapidly wobbled, provided the wobble 
does not cover too great a frequency range. 
Such wobbles are sometimes used in combina¬ 
tion with simple inversion, to prevent reinvert¬ 
ing with a locally supplied carrier at the edge 
of one sideband. 

Spread-Band. In this system, some or all of 


the sidebands are displaced from the carrier. 
Demodulated signals of this type will cover an 
a-f range greater than 3 kc, usually as high as 
6 kc. It is essential, therefore, that the receiver 
be capable of handling such a band. To obtain 
the intelligence, the signals must be further 
demodulated (B1 in Table 1, page 47). 

Suppressed Carrier. In the ordinary trans¬ 
missions described above, the carrier level is 
high compared to the speech sidebands. To 
avoid loading up the transmitter with carrier, 
and thereby permit radiating a higher sideband 
level, many channels operate on the “sup¬ 
pressed carrier" basis. In this system the carrier 
is either eliminated completely, or transmitted 
with greatly reduced level. To demodulate such 
signals properly, the weak carrier must first be 
greatly amplified, or a new one supplied locally. 
If this is not done the signals will demodulate 
themselves around whichever component in the 
sideband happens to be predominant, produc¬ 
ing thoroughly scrambled speech which can 
thereafter not be restored. This condition can 
be recognized by its characteristic sound to the 
ear, together with wide syllabic fluctuations of 
the meter which ordinarily indicates the carrier 
level. 

Twin Channels. With suppressed carrier sys¬ 
tems, usually only one of the speech sidebands 
is transmitted. However, a second sideband, 
transmitting a second speech channel, is some¬ 
times added, usually displaced from the carrier 
by about 3 kc, to avoid crosstalk between the 
channels. This is called “twin-channel" opera¬ 
tion, and gives on demodulation an audio signal 
covering about 6 kc. The two channels must be 
separated and placed in their normal positions 
by the methods cited under spread-band sys¬ 
tems. 

The above systems are the main types of 
radio transmission used commercially with am¬ 
plitude modulation. In addition, in the v-h-f 
range and above, there are frequency-modula¬ 
tion systems, and also pulse-modulation sys¬ 
tems, both of which require receivers specially 
designed to handle their particular types of 
signals. This is too large a subject to cover 
here, and reference must again be made to the 
radio literature. 

Finally it should be mentioned that in addi- 




INTERCEPTION 


39 


tion to speech a great deal of telegraph trans¬ 
mission will be found. There are several types 
of telegraph signals, including hand-keyed, 
such as Morse code, or machine-keyed such as 
Boehme and teletype. Any of these types may 
be transmitted by keying the carrier, or by key¬ 
ing a tone modulated on the carrier. The marks 
and spaces may be represented by changing the 
amplitude (on-off) or by changing the fre¬ 
quency (two-tone). Finally, since telegraph re¬ 
quires a much smaller band than speech, it is 
often operated on a multichannel basis, that is, 
a voice channel will be divided into a number of 
telegraph channels. In addition, there are fac¬ 
simile transmission systems, which also may 
be operated on an a-m or f-m basis. If a new 
signal is encountered whose nature is in doubt, 
these possibilities should be kept in mind for 
further investigation when the need arises. 

Recording 

The same considerations which make it de¬ 
sirable to obtain a good intercepted signal, apply 
also to recording and reproducing scrambled 
speech. In addition to the requirements as to 
quality and noise, there is an even more serious 
one concerning speed regulation. In general, 
systems designed for a high degree of privacy 
require a high degree of synchronization, and 
in many cases ordinary recording methods are 
not good enough, not only in long-time average 
speed regulation, but also in the steadiness of 
the instantaneous speed. In the case of some of 
the systems the requirements are so severe that 
even the best commercial recorders will not 
meet them. 

The best solution of this problem is to decode 
before recording. This will be possible in many 
cases, although it may sometimes entail the 
loss of parts of the message while adjustments 
are being made or the code is being deter¬ 
mined. It happens that some systems which im¬ 
pose the severest requirements on speed regu¬ 
lation (B3 in Table 1), can be handled in this 
way. When this method is feasible, even poor 
quality recorders, such as those designed to 
record a great deal of material in a small area, 
may be good enough. 

In some of the systems it will not be possible 
to decode before recording. It happens, however, 


that in the case of the only known system for 
which this is true (F3 in Table 1), the require¬ 
ments as to quality and speed can easily be met 
by good commercial type recordings. 

The matter of convenience or ease of use of 
the reproducing system is very important in 
decoding work. In this respect also, the require¬ 
ments are different for different privacy sys¬ 
tems. The recording systems using the emboss¬ 
ing process, for instance, are convenient 
because they produce no thread, and they re¬ 
quire little attention. However, they all suffer 
from poor tracking during reproduction, which 
can be exceedingly burdensome, especially 
where the material must be reproduced many 
times over. Recording magnetically on wire is 
attractive from the standpoint of convenience 
and also quality, but back-tracking is very time- 
consuming and laborious. 

The best solution, at the present writing, ap¬ 
pears to be disk recording on acetate, with a 
machine capable of recording at various speeds. 
Low speeds can be used where quality need not 
be too good, and a long record is desired. Higher 
speeds can be used where better quality is 
needed. Such recording systems are commer¬ 
cially available. 

Decoding Tools 

In addition to the facilities discussed above, 
an intercept station, if it is to be prepared to 
diagnose and decode intercepted enemy signals, 
must be equipped with a considerable variety of 
special tools. These should include such well- 
known devices as oscilloscopes, amplifiers, oscil¬ 
lators, modulators, rectifiers, fixed and variable 
filters, and a supply of components for con¬ 
structing special circuits that may be required. 
Some of the less well-known devices include 
magnetic tape or wire recording and reproduc¬ 
ing equipment in the form of loops with mul¬ 
tiple pickups, commutators for sweep or timing 
circuits, variable-speed drive mechanisms, chan¬ 
nel shifters, the variable-area pattern machine, 
and the sound spectrograph. There should also 
be models of the more important types of exist¬ 
ing speech privacy systems. Finally, and per¬ 
haps most important of all, there should be sta¬ 
tioned at the intercept location a group of 
highly trained technicians, who should be thor- 



40 


UNSCRAMBLING AND DECODING METHODS 


oughly familiar with radio transmission prob¬ 
lems, radio facilities, cryptanalytic procedures, 
and diagnosing and decoding methods. If these 
technicians are not conversant with the lan¬ 
guage encountered in intercepted communica¬ 
tions, interpreters should be continuously avail¬ 
able. 

Even with all the special tools and personnel, 
decoding in many instances will be a difficult 
problem, and patience and painstaking effort 
will be required to obtain useful information 
from scrambled speech. Unless the needs have 
been anticipated the enemy may have secret 
communication for a considerable period of time 
as a direct result of unpreparedness. 


4 3 NONCRYPTOGRAPHIC TOOLS AND 
METHODS 

Beginners in the study of privacy systems 
never fail to be amazed at the difficulty of 
scrambling speech sufficiently to destroy the in¬ 
telligence. The ear can tolerate or even ignore 
surprising amounts of noise, nonlinearity, fre¬ 
quency distortion, misplaced components, gaps, 
superpositions, and other forms of interference. 
Very often partial or even complete intelligence 
can be obtained from a privacy system by par¬ 
tial or imperfect decoding, and this in turn can 
often be accomplished by operating on the 
scramble in some way which the designer did 
not contemplate. 

The fact that the ear is such a good decoding 
tool in combination with noncryptographic 
methods makes the production of privacy sys¬ 
tems very difficult. Scrambling systems which 
look very effective on paper sometimes turn out 
on trial to degrade the intelligibility very little, 
although the scrambled speech usually sounds 
unpleasant. Most methods pushed to the point 
where they succeed in hiding the intelligibility 
so distort the speech that it is impossible to 
restore the speech with good quality. In fact, 
there are very few speech privacy systems 
which achieve a high degree of privacy with 
acceptable quality. 

Noncryptographic methods are very impor¬ 
tant, because they may reduce the delay in ob¬ 
taining the intelligence substantially to zero. 


Furthermore, they may render completely futile 
the most elaborately irregular code changing 
systems which could be handled only with the 
greatest difficulty by straight cryptographic 
methods. A number of noncryptographic meth¬ 
ods are given below. Some of them, of course, 
result in poor quality, but the saving of time, 
labor, and equipment may be very great. 

Captured Set or Functional Equivalent 

With many privacy systems all that we need 
to listen in is a captured set or its functional 
equivalent built from knowledge of the scram¬ 
bling method. An extreme example of this is 
simple inversion. In this case the scrambled 
speech is quite unintelligible to direct listening, 
but if we know it is inversion, we can find the 
inversion frequency very quickly by trial. An¬ 
other example is the split-phase system (A5). 
The phase-shifting network in the captured set 
could readily be adjusted to demodulate either 
of the two overlapping sidebands. 

Slightly more complicated systems are those 
with a simple program. Again with a captured 
set or its equivalent it is usually easy to find the 
program by trial. The only possible difficulty is 
in keeping step with the sending end, particu¬ 
larly if there is no synchronizing pulse. An 
example of this is a wobble band displacement 
(B3). If, for instance, the wobble is sinusoidal, 
with the frequency and the sweep limits known, 
the problem is to keep in synchronism. In this 



Figure 1. Method of aided tracking. 


connection “aided tracking’' might be men¬ 
tioned, a device which is familiar in gunfire 
control circles. With this system changes in 
both frequency and phase are made simultane¬ 
ously. This is illustrated in Figure 1. Suppose we 













NONCRYPTOGRAPHIC TOOLS AND METHODS 


41 


find ourselves slightly out of step with the sig¬ 
nal. By rotating the adjusting handle forwards 
or backwards we can get back into step. Suppose 
this adjustment was in the forward direction. 
The fact that we had to catch up is an indica¬ 
tion that the motor is slow. Therefore, some of 
the motion of the handle required for catching 
up is used by means of gearing to change the 
frequency driving the motor. The gear ratios 
are chosen to suit the particular problem. With 
this method it is possible to get into step with 
and stay in step with systems such as alternate 
displacements and regular wobbles. 

Compromise Decoding Methods 

All the methods outlined in this section have 
been tried, at least in the laboratory. Their suc¬ 
cess, however, naturally depends to some extent 
on the switching rates and similar variables. It 
is possible, therefore, that a method might 


MOD - 

BP FILTER 

- UPPER CUTOFF 13 KC 

LOWER CUTOFF 11.5 TO 12.9 KC 

- MOD 

I- 


3b 


Figure 2. Band-shift filter; an important decod¬ 
ing tool. 


prove unsuccessful against a scrambling system 
which seems to be in the same general class as 
the one that was tried in the laboratory. 

Consider, for example, a system (A2) which 
involves inversion about a number of frequen¬ 
cies in succession. If these frequencies are not 
too far apart we can choose a single frequency 
somewhere in the middle range and demodulate 
the whole signal with this one frequency. The 
resulting band will be right side up, but dis¬ 
placed by varying amounts not exceeding half 
the total range. This has been found to be quite 
intelligible, provided the switching rate is not 
too high or the range of frequencies too wide. 

With some systems it is expedient to listen 
to only a portion of the frequency range rather 
than the whole range. An outstanding example 
of this is the system in which the subbands are 
variously delayed (FI). Conceivably, these de¬ 
lays could constantly be changed with time ac¬ 
cording to a never repeating program. This, 


however, would be futile because with a band 
filter we need listen to only one of the bands, 
disregarding the others. Unless this band is 
very narrow the intelligibility may be practi¬ 
cally complete. Similarly in band-splitting sys¬ 
tems if the switching is not rapid (Dl) we can 
follow one of the bands around the frequency 
range. The lowest or second lowest band is usu¬ 
ally the best. Another example is the tone se¬ 
quence (J3); instead of trying to filter out one 
tone at a time as it occurs, we can leave all the 
filters in all of the time and still have enough 
speech coming through to yield the intelligence. 

A special case in which the rejection of a 
part of the frequency band of the scramble 
makes decoding easier concerns those systems 
such as A5 which depend on carrier phase to 
mix and then separate components. There is no 
phase requirement imposed on the demodulat¬ 
ing carrier unless both sidebands are trans¬ 
mitted. Therefore, either sideband of such a 
system may be suppressed with a filter, and the 
remaining sideband demodulated with a carrier 
of any phase. The two signals in the sideband 
will then be simply superposed. 

For purposes such as those outlined a valu¬ 
able tool is the band-shift filter illustrated in 
Figure 2. With this device a band of adjustable 
width can be taken from any portion of the 
signal frequency range (0 to 3 kc) and relocated 
in any other portion of the same frequency 
range either straight or inverted. One form of 
band-shift filter is described in Preliminary Re¬ 
port No. 11 of Project 0-43.^® It consists essen- 


AIR CONDENSERS 



Figure 3. Band-pass filter in which pass band is 
variable. 


tially of a double modulator, but with a band 
filter of variable width. If the frequency loca¬ 
tion of the band is not to be changed, the switch 
in Figure 2 should be in the left-hand position. 
One form of variable band filter is shown in 
Figure 3. This tool has also proved useful in 













42 


UNSCRAMBLING AND DECODING METHODS 


certain other systems such as the multiplica¬ 
tion system (HI) and the time division multi¬ 
plex [TDM] system (El). 

Sometimes it is expedient to listen to a scram¬ 
ble only part of the time. Some of the simpler 
coding programs can sometimes be broken down 
in this manner by trial. For instance, if a cod¬ 
ing cycle has N elements we can listen to every 
Nth element and make whatever adjustments 
are needed to make this sound natural. We can 
then listen to the next adjacent element and 
adjust the system so that these elements blend 



A B 


PEAK CHOPPERS 



Figure 4. Elementary circuits of peak choppers 
and compressor. 


vices should be useful against any privacy sys¬ 
tem in which sudden changes of level occur. A 
good example is the subband level modulation 
system (H3). A separate limiter or compressor 
in each of the subbands will tend to smooth 
out the level variations and make the speech 
intelligible. 

Another nonlinear device is the rectifier. Two 
forms are shown in Figures 5A and B. The 
rectifier as used here should not be confused 
with the detector. The latter device also recti¬ 
fies, but it then has a time constant incorporated 
in the output circuit which tends to smooth the 
output and give the envelope wave. The rectify¬ 
ing action which is wanted here simply takes 
all the negatives lobes of the signal and turns 
them over. As in the case of the limiter, straight 
speech put through a rectifier of this type is 
about 95 per cent intelligible. 

In the privacy system designated A4 the 
phase of the speech signal is reversed at short 
irregular intervals. If this signal is now recti¬ 
fied, all the negative lobes will be made positive 
and the resulting wave will be indistinguishable 
from rectified straight speech except for slight 
discontinuities at the points where the reversals 



properly. This attack applies to a system in 
which several different displacements are used 
(B2). A captured set, of course, is the easiest 
way of selecting every Nth element because it 
is usually easy to make the other time elements 
inoperative. 

Another useful device is the limiter, or peak 
chopper. In this same class is the compressor. 
These are illustrated in Figures 4A, B, C. They 
all tend to equalize the successive lobes of a 
complex wave. The peak chopper simply chops 
off any peak which exceeds a certain instan¬ 
taneous voltage. The compressor operates more 
gradually and leaves the waves well rounded. 
If straight speech is put through any of these 
devices, distortion products are generated be¬ 
cause the wave form is radically modified. It is 
found, however, that this kind of distortion 
damages the intelligibility very little. These de¬ 



Figure 5. Two forms of rectifier circuits. 

occurred in the privacy system. This is illus¬ 
trated in Figure 6. Therefore, a simple phase- 
reversal system, no matter how irregular, 
should yield to rectification except that distor¬ 
tion in the transmission process tends to change 
the wave form and thereby degrade the quality 
of the resulting speech. It should be noted that 
the multiplication process (HI) also results in a 
phase reversal every time the coding wave 
passes through zero. It has been found that rec¬ 
tification tends to make this kind of scramble 
more intelligible also. 
























NONCRYPTOGRAPHIC TOOLS AND METHODS 


43 


A very useful noncryptographic device is 
superposition. For instance, with a three- 
channel mixing system such as LI or L2, if 
all three channels are listened to simultaneously, 
three conversations will be heard at once, or 
possibly one conversation with two noises super¬ 
posed. Experience has shown that under such 
conditions it is usually easy to concentrate on 
the desired channel and ignore the others. 

Another form of superposition is illustrated 
by the following: Consider a split-band system 




I I 

I I 

I I 

' ^ 


! I 




I I 




I I I 


ORIGINAL SIGNAL 


PHASE REVERSED 


ORIGINAL RECTIFIED 


tions is made from each of the band-pass filters 
to the output modulators whereby each of the 
bands in the signal is placed in the desired bands 
in the output. Steps should be taken to see that 
these cross connections do not interfere with 
each other. An amplifier after each band filter, 
for instance, will perform this function. Figure 7 
illustrates a simpler case of superposition ap¬ 
plied to a system using two band shifts (B2). 

It may be noted here that superposing time- 
displaced elements does not appear to be suc¬ 
cessful. For instance, if all the segments of the 
commutator in a time division scrambling 
[TDS] machine are connected to all the pole- 
pieces, the output will be straight speech with 
several scrambles superposed. This has been 
found to be completely unintelligible. 

In certain cases which have been met in 
Project C-43 the privacy sets are equipped with 
dials or similar means which were intended to 
provide an easy method for obtaining a large 
number of different codes. In some cases the 
different codes may not be sufficiently different 
to be mutually private. That is, while there may 
be literally millions of different combinations, 
it sometimes happens that there are thousands 


Figure 6. Action of rectifier, useful in breaking 
down multiplication scrambles. 


(D2) in which six different codes are used in a 
never repeating sequence. This would be rather 
difficult to handle by cryptographic means. Sup¬ 
pose, however, we had six separate decoding 
units, each set to decode one of the six codes. If 
the scrambled signal were fed into all six of 
these decoders simultaneously, one of them 
would always have straight speech in its output. 
The other five would be scrambled. If these six 
outputs are all superposed, we will hear straight 
speech with five scrambled superposed. This 
straight speech can be understood quite easily. 
It will be noted that the unwanted components 
in this kind of superposition are derived from 
the wanted components, and always vary in 
level simultaneously with the wanted compo¬ 
nents; it appears that under these conditions 
they do not do much damage. 

The split-band equipment illustrated in Fig¬ 
ure 6 of Chapter 1 is adapted for this kind of 
superposition. A multiplicity of cross connec¬ 



Figure 7. One form of superposition decoding. 


of combinations which will decode material 
scrambled with one of the combinations. Vari¬ 
ous degrees of quality will result from these 
partial or incorrect decoding operations. How¬ 
ever, as long as intelligibility can be extracted 
the codes cannot be considered mutually private. 
In such cases it is possible with a captured 
machine simply to manipulate the dials sys¬ 
tematically or unsystematically and listen to 
the result. When the speech begins to sound 
somewhat natural, systematic trials of each dial 
in turn will sometimes steadily improve the 
quality. Something of this sort could be done 




















44 


UNSCRAMBLING AND DECODING METHODS 


with simple TDS systems also, except that the 
use of interlaced codes makes this somewhat 
more difficult. 

In certain cases where there are a large 
number of codes possible but only a few of these 
codes are good codes from the standpoint of 
direct listening, it would seem reasonable that 
any code applied to the scrambled signal should 
turn the good code into a poor code. In the five- 
band split-band system for instance, there 
are some 3,840 possible codes but only twelve 
or so are considered really good. Any code in 



Figure 8. Use of directional discrimination 
against noise. 


the decoding machine, therefore, should de¬ 
crease the privacy for direct listening. This has 
been tried in the laboratory but has not been 
pushed to the point of determining whether it 
could compete with the superposition method. 
The idea may possibly apply to other systems 
which may be encountered. 

A very specialized device, which applies to 
wire line communication only, should be men¬ 
tioned here because it is not very well known. 
It distinguishes between the two directions of 



Figure 9. Use of relay to disconnect receiver 
from line during heavy bursts of noise. 


transmission over wires. In the masking pri¬ 
vacy system J2, for instance, the clear signal 
in one direction is masked by noise sent in the 
other direction. The device illustrated in Fig¬ 
ure 8, however, discriminates against the noise, 
allowing the speech to be heard. It requires a 
small series resistance, which is built up by a 
step-up transformer to the line impedance. The 
secondary is connected to the other side of the 


line. The direction of discrimination depends on 
the phasing of the transformer windings. 

Automatic Decoding 

Whether speech is intelligible or unintelli¬ 
gible is purely a subjective matter. However, 
the method of making speech unintelligible in¬ 
volves making physical changes in the speech 
wave. Certain kinds of physical changes can 
be detected quite readily by objective means 
and utilized to undo the scramble automatically. 
The most elaborately irregular code program 
is completely futile if this kind of decoding can 
be applied. 

A very simple example of this is shown in 
Figure 9. Suppose the system consists of short 
spurts of noise applied in an irregular manner. 
The noise must be high in level compared to the 
speech in order to mask the speech. Therefore, 
if the signal is applied to an amplifier-detector, 
connected to a relay (or electronic switch) the 
relay can be so biased that it operates only on the 
noise spurts. The receiver is momentarily dis¬ 
connected from the line whenever a noise spurt 
occurs. The same method can be used for level 
modulation systems (H2 and H3). Instead of 
disconnecting the receiver, the high-level por¬ 
tions of the signal cause the receiver to be con¬ 
nected to a parallel path containing the required 
amount of loss to equalize the levels. In the case 
of subband level modulation (H3), a separate 
device of this type must be used in each sub¬ 
band. 

The system just described operates on a total 
energy basis. Sometimes it is possible to obtain 
a switching signal on the basis of energy fre¬ 
quency distribution. Consider, for instance, the 
system using two different displacements (B2). 
The alternate positions of the speech band are 
illustrated in Figure 10. In one position, the 
band is right side up and occupies the range 
from 2 to 5 kc. The alternate position is in¬ 
verted occupying the range from 3 to 6 kc. 
Since most of the energy in the speech band 
is concentrated in the low-frequency part of the 
original spectrum most of the time, the system 
illustrated in Figure 11 can be used to decode 
this material automatically. The signal is ap¬ 
plied to two band filters, one covering the range 
2 to 3 kc, the other passing 5 to 6 kc. The out- 

















NONCRYPTOGRAPHIC TOOLS AND METHODS 


45 


puts of these band filters are rectified individu¬ 
ally and fed to the two windings of a polar 
relay. The relative energy in the two band 
filters will be different for the two displace¬ 
ments and the relay in Figure 11 will be oper¬ 
ated alternately in the two directions, thereby 
automatically connecting the proper carrier to 
the input modulator in Figure 7 to put the 
speech band in its normal position. This will 
not be infallible, but with displacements as 
different as the ones used in the illustration, it 
should operate sufficiently well to yield most of 
the intelligence of the message. Naturally, the 
smaller the physical difference between the two 
positions being distinguished, the more false 
operations there will be. However, this method 
is instantaneous even with an irregularly 
switched system, whereas cryptographic meth¬ 
ods would be very time-consuming. 

Another variation of this general technique 
might be mentioned for the sake of complete¬ 
ness although it is somewhat more speculative. 
Consider a privacy system which depends on 
speed changes (F4). Changes in speed cause 
changes in the pitch of the voice. Suppose we 
apply this signal to a circuit which measures 
the voice pitch. This technique has been worked 
out in connection with the Vocoder. The output 
of this circuit, which is a varying frequency, 
is used to change the speed of a motor. The 
motor is part of the drive of a magnetic tape 
recording and reproducing system through 
which the signal is passed. As the motor speed 
is made to change, the tape speed changes in 
such a direction as to tend to keep the derived 
frequency constant. This takes out not only the 
speed variations, but also the voice inflections. 
However, a monotone is quite intelligible. 

The following method, which has not yet 
been tried out, is intended to apply to irregular 
band displacements or wobbles (B4), which 
would be exceedingly difficult to handle any 
other way. Consider a system in which the band 
is kept right side up, but is wobbled over a 
range sufficient so that demodulation with some 
intermediate carrier frequency will not give an 
intelligible signal. Suppose the wobble follows 
an irregular, nonrepeating program. The fol¬ 
lowing decoding method is proposed. 

The signal is impressed on a network having 


a very steeply rising loss characteristic. If the 
speech band were not wobbled, this network 
would simply tend to make the lowest harmonic 
of all voiced sounds the strongest component. 
With the wobble, the same thing will be true 
except that the level of this component will 


TIME 



0 5KC 10 KC 

FREQUENCY 

Figure 10. Sidebands in two-position displace¬ 
ment system. 

undergo severe fluctuations. Therefore, the re¬ 
sulting signal is subjected to some form of 
automatic volume control and also a limiting 
action, tending to derive a single frequency. 
Forgetting voice inflections for the moment, 
this derived frequency would fluctuate up and 
down (in frequency) in step with the band 
wobble. In fact, it could be used as a subcarrier 
in a double modulation decoder to demodulate 
the signal to approximately the correct position 
in the frequency range. It will be in error, 



Figure 11. Automatic decoding system depend¬ 
ing upon energy-frequency distribution. 


however, by an amount equal to the instantane¬ 
ous voice pitch. Possibly this amount of error 
will not prevent the signal from being intelli¬ 
gible (we know that this amount of displace¬ 
ment does not destroy the intelligence of other¬ 
wise normal speech). 

If it is desired to correct for this error, two 
methods suggest themselves. One possible 
method is to subtract from the derived fre¬ 
quency, by a modulation process, an amount 
















46 


UNSCRAMBLING AND DECODING METHODS 


equal to the average pitch of the voice being 
monitored. This will leave a small fluctuating 
error. Another possibility is to derive the actual 
instantaneous voice pitch, by difference tone 
methods, and subtract this amount from the 
derived subcarrier frequency. 

If the displaced band is inverted instead of 
right side up, a similar procedure can be used, 
with a network of opposite loss characteristics. 
In either case this method will correctly demod¬ 
ulate only the voiced sounds, but experience 
suggests that this is sufficient. If not, some kind 
of carry-over effect might be incorporated in 
the system to prevent sudden changes in the 
subcarrier frequency, thereby tending to hold 
over correct demodulation for short unvoiced 
sounds also. This method has not been tried, 
but is felt to be worth recording because of the 
great difficulty of handling irregularly wobbled 
systems by any other method. 

Another rather speculative automatic method 
might be mentioned because some form of it 
might prove useful against certain multiplica¬ 
tion systems such as HI. The code wave in the 
particular case encountered was repeated many 
times per second, and there was a synchroniz¬ 
ing pulse ahead of each cycle. If the signal is 
applied to a synchronized cathode-ray oscillo¬ 
scope with a highly persistent screen, a definite 
pattern appears because the coding wave al¬ 
ways passes through zero at the same time. 
Also, the speech energy tends to average out 
after a few cycles so that the pattern reflects 
the amplitude of the coding wave. It is quite 
conceivable that this pattern on the screen 
could be scanned optically and used to generate 
a decoding wave for automatically unscram¬ 
bling the signal. Obviously, if the coding wave 
is changed periodically, a new decoding wave is 
automatically produced. The only requirement 
is that the coding wave persist long enough to 
form an average pattern on the screen. 

Another variation of automatic decoding 
methods might be termed ‘‘parallel-automatic” 
because two or more complete decoding units 
are used in parallel but only the correct one is 
applied to the listening receiver. To emphasize 
the difference between this method and the one 
previously discussed, we will use the same ex¬ 
ample, namely, the system with two band dis¬ 
placements. Referring to Figure 7, suppose 


instead of the parallel modulators, there were 
two complete units in parallel including the 
band filter, the second modulator, and the out¬ 
put filter. One of the units is fed with the 8-kc 
carrier, the other with the 16-kc carrier. Each 
unit will have straight speech in its output half 
the time, and the other half the time will have 
inverted speech displaced by 1 kc. A 1-kc low- 
pass filter can then be used in a device similar 
to Figure 11 to switch the listener to whichever 
one of the decoding units has the straight 
speech. For the particular system used in the 
illustration, there does not appear to be any 
particular advantage of one method over the 
other. However, the latter system can be ap¬ 
plied in cases where the other method might 
not be feasible. 

The parallel-automatic method can be made 
to give a different type of switching signal. For 
instance, use might be made of the harmonic 
relationship between the components of speech 
when the speech band is in its normal position. 
If the voice pitch happens to be 100 cycles, then 
all the harmonics will be multiples of 100 cycles. 
If this speech is put through a suitable non¬ 
linear system such as a rectifier or limiter, 
difference tones will be generated which will 
also be multiples of 100 cycles. If, however, the 
speech band is displaced from its normal posi¬ 
tion in any way, the difference tones will not 
coincide with the speech components. If, for 
instance, the whole band has been displaced by 
50 cycles, then the speech components will be 
150, 250, 350 cycles, etc. The difference tones 
generated by a nonlinear system will be 100, 
200, 300 cycles, etc. If we now take a second 
difference between the output of the nonlinear 
system and the original components, there will 
be generated multiples of 50 cycles. The lowest 
component of this series will be lower than the 
pitch of the voice. This will be true regardless 
of how far the original band has been shifted, 
except for the special case where the shift 
happens to be an exact multiple of the voice 
pitch. Since, however, the pitch is constantly 
varying, this coincidence is of very brief dura¬ 
tion. Theoretically, at least, a low-pass filter 
with a cutoff lower than the normal range of 
voice pitch can be used as a clue to determine 
whether a speech band is in its proper location. 
The method then would consist in having sev- 



NONCRYPTOGRAPHIC TOOLS AND METHODS 


47 


eral decoders in parallel, listening only to the 
one which did not generate a component in the 
low-pass filter. 

The above illustrations will serve to show 
the possibilities of noncryptographic types of 
attack on privacy systems. When a new system 
is encountered, this type of attack should be 
given serious consideration because of the 


saving in time and equipment. Naturally, as 
pointed out above, straightforward crypto¬ 
graphic attack can be made to yield a bet¬ 
ter quality signal. However, experience has 
shown that the ear can become familiar with 
certain kinds of distortion and learn to extract 
the intelligence more and more readily with 
practice. 


Table 1. Summary of scrambling systems. 


A. Single modulation 

1. Inversion 

2. Variable frequency in¬ 

version 

3. Alternate inversion 

4. Phase reversal 

5. Split phase 

B. Double modulation 

1. Fixed displacement 

2. Stepped displacement 

3. Wobbled displacement, 

regular 

4. Wobbled displacement, 

irregular 

C. Triple modulation 

1. Re-entrant inversion, 

steps 

2. Re-entrant inversion, 

continuous 

D. Band splitting 

1. Slowly switched 

2. Rapidly switched 

E. Time division multiplex 

1. 4-band system 

2. With noise channel 

F. Magnetic tape 

1. Delayed subbands 

2. TDS, repeated code 

3. TDS, nonrepeated code 

4. Speed variations 

5. Backwards trans¬ 

mission 

6. Alternate backwards 

and forwards 


Noncrypto¬ 
graphic 
attacks 
(Table 2) 

Capable of 
crypto¬ 
graphic 
attacks 


Noncrypto¬ 
graphic 
attacks 
(Table 2) 

Capable of 
crypto¬ 
graphic 
attacks 



G. Tape plus modulation 



la 


1. TDS and inversion 

2f, 2g, 2h 

* 



2. TDS and split band. 



lb, 2a, 2f 

.... 

synchronous 


* 

lb, 2f 

• • • • 

3. TDS and split band. 



lb, 2e, 2g, 3e 

❖ 

nonsynchronous 


* 

la 


4. Two-dimensional 


* 



5. Double-speed split 

lb 

.... 

la 

• • • • 

6. Half-speed split 

lb, 2c 

.... 

lb, 2a, 2f, 3b, 3f .... 

H. Wave form distortion 





1. Multiplication 

2b, 2e, 2g, 

3e * 

lb, 2a 

.... 

2. Level modulation 

2d, 3a 

* 



3. Subband level modula¬ 



3d 

* 

tion 

2d, 3a 

* 



J. Masking methods 





1. Signal plus noise, same 



lb, 2c, 2f 

.... 

direction 


* 



2. Signal plus noise. 



lb, 2a 


opposite direction 

2i 

.... 



3. Tone sequence 

lb, 2b, 3a, 

3b 

lb, 2b, 2f, 2g, 2h 

4. Noise spurts 

2c, 2d, 3a 

.... 

2c, 2f, 2h 


5. Nonlinear distortion 

la 

.... 

lb, 2b 


K. Vocoder methods 



lb, 2b 


1. Permute channels 


* 



2. Invert channels 

lb, 2g 

* 

2b 


3. TDS channels 

lb, 2g 

* 

Op Orr 9h 

* 

4. Two-dimensional 




* 

scramble 

lb, 2g 

* 

lb, 2a, 3c 

* 

5. Time division multiplex lb 

* 



L. Channel mixing 



lb 

.... 

1. Time division mixing 

2c, 2f 

* 



2. Subband mixing 

2b 

* 

lb, 2c 

.... 

3. Combination 

2c, 2f 

* 


Table 2. Noncryptographic decoding methods. 


1. Captured set or functional equivalent 

a. Fixed condition—find by trial 

b. Simple program—get into step 

2. Compromise decoding methods 

a. Intermediate condition 

b. Listen to portion of frequency band 

c. Listen part time 

d. Limiter, peak chopper, compressor 

e. Rectifier 

f. Superposition 

g. Approximate code by trial 


2. (Continued) 

h. “Spoil” good code by recoding 

i. Directional discriminator 

3. Automatic decoding 

a. Total energy 

b. Energy frequency distribution 

c. Pitch-change corrector 

d. Wobble corrector 

e. Code wave generator 

f. Parallel automatic 

g. Inharmonic detector 


rSECRETlII^ 




























48 


UNSCRAMBLING AND DECODING METHODS 


In general, noncryptographic methods re¬ 
quire that the signal, as received, be of fairly 
good quality. In some cases, the saving in time, 
labor, and equipment would be so great that if 
the signal, as received, is too poor to permit 
noncryptographic attack, the most reasonable 
thing to do is to move the intercept station to 
get a better signal. 

In Table 1, there is listed for each privacy 
system, the type of noncryptographic attack 
which might apply. It should be emphasized 
once more, however, that the method which 
succeeds at one switching speed may fail at 
another. The list, therefore, should be taken 
only as a recommendation of systems which 
should be considered. The noncryptographic de¬ 
coding methods are summarized in Table 2. 

4 4 CRYPTOGRAPHIC TOOLS AND 

METHODS 

A cryptographic decoding method involves 
(1) actually determining a code which will 
undo the scramble, and (2) restoring the 
speech by means of this code. In the case of 
repeated codes, this can sometimes be done 
rather quickly. An example is the repeated-code 
TDS system. The actual codes used can be 
found in about 15 min. Having found the code, 
we can set it into our receiving machine and 
thereafter listen to the speech directly. In the 
case of nonrepeated codes, every bit of the 
message must be handled individually. It may 
take a thousand or even a hundred thousand 
times as long to decode as it did to speak the 
words. It may take hours or even days to obtain 
the intelligence from a short message; mean¬ 
while other messages will have been sent and we 
get farther and farther behind. The only way 
this could be avoided would be to have approxi¬ 
mately as many teams working in parallel as the 
ratio of decoding time to message time, which, 
of course, is impractical if the ratio is large. 

As stated previously, the sound spectrograph 
—described in detail later—is of tremendous 
assistance in recognizing the nature of an un¬ 
known scrambling system. The ear can usually 
recognize the presence of time discontinuities. 
It can also recognize the peculiar quality which 
results from band-shifting systems. The exact 


nature of the scramble, however, is usually 
impossible to establish with the ear. Even 
scrutiny of the wave form may yield no clue. 
The strikingly graphic analysis provided by 
the spectrograph, however, usually takes the 
mystery out of the scrambling method immedi¬ 
ately. 

For example, speech privacy systems having 
frequency subbands will show horizontal dis¬ 
continuities or boundaries in their spectro¬ 
grams. Similarly, systems employing time 
division will show vertical boundaries. A con¬ 
siderable variety of systems display both hori¬ 
zontal and vertical boundaries. Methods of 
telling these systems apart are described in the 
final report on Project C-32.44 

Program Determination 

The simplest cases to handle are those in¬ 
volving a program which can be determined 
directly from spectrograms by inspection or 
measurements. The re-entrant inversion system 
(Cl) might be used as an example. Suppose a 
multiplicity of displacements were used in some 
irregular sequence. Discontinuities marking the 
inversion frequencies appear in the spectro¬ 
grams and once they have been determined by 
measurements on a large number of spectro¬ 
grams, the program can thereafter be deter¬ 
mined quite readily by using a template. This 
template can be marked directly with the set¬ 
tings of the decoding machine which will restore 
the speech to its normal position. 

Another example involving a program would 
be one like B2, in which two different displace¬ 
ments are used alternately, with the intervals 
irregular in duration. Here the time boundaries 
will be quite apparent and they can be measured 
with a suitable time scale. 

In all likelihood changes of the above types 
will occur in discrete steps for practical reasons. 
The use of a program involving continuous 
changes with time presents formidable tech¬ 
nical difficulties at the authorized as well as the 
unauthorized terminals. 

Matching Spectrograms 

In cases where the scrambling system in¬ 
volves rearrangement of the speech elements 
in time or in both time and frequency, the basic 



CRYPTOGRAPHIC TOOLS AND METHODS 


49 


method for determining the codes involves cut¬ 
ting up spectrograms along the element bound¬ 
aries and rearranging the elements so as to 
restore the straight speech. An example is 
shown in Figure 12. The criterion for rear¬ 
ranging the elements is that there should be 
continuity at the boundaries. This continuity 
includes the position and direction of the indi¬ 


means for making a mechanically inverted pat¬ 
tern as well as a normal pattern. The spectro¬ 
gram at the top of Figure 13 shows a normal 
pattern. Directly below it is an inverted pat¬ 
tern of the same material. A mechanically in¬ 
verted pattern is indistinguishable from a 
pattern produced by electrical inversion of the 
speech. Similarly, if the whole inverted spectro- 



Figure 12. Method of matching spectrograph patterns of nonrepeated code TDS. 


vidual harmonics, the position and direction of 
the resonance areas, and, in general, the ampli¬ 
tude as represented by the darkness or lightness 
of the patterns. The pieces are numbered before 
the matching process begins and when the 
matching has been completed, the numbers on 
the pieces determining the code. 

If the scrambling process involves inversion 
of the time or frequency scales, straight speech 
can be restored for matching purposes by mak¬ 
ing two spectrograms as shown in Figure 13. 
Present models of the spectrograph include 


gram is turned through 180 degrees, so that the 
base line is at the bottom and towards the ob¬ 
server, the result is indistinguishable from the 
case in which the speech is transmitted back¬ 
wards. Therefore, if an element in the scram¬ 
ble is inverted, it may be recovered as straight 
speech for matching purposes by cutting the 
element from the mechanically inverted pat¬ 
tern. If an element has been transmitted back¬ 
wards, it can be restored to normal by cutting 
it from the inverted pattern and rotating it 180 
degrees as described above. If it is both back- 










































50 


UNSCRAMBLING AND DECODING METHODS 


wards and inverted, it may be restored by cut¬ 
ting it from the regular pattern and turning it 
around. 

It has been found from experience that 
matching is facilitated by enlarging the spectro¬ 
grams by a factor of about two to one. Not only 
is the increased size easier to handle, but the 
heavy photographic paper is much better to 
handle than the facsimile paper employed in the 


poses regularly, then it may pay to adopt the 
technique described in Preliminary Report No. 
13^'^ (Project C-43) for producing large spectro¬ 
grams photographically. 

To facilitate matching, appropriate means 
should be used for handling the elements. It 
has been found that a slightly adhesive surface 
is advantageous. In the illustration of Figure 12 
this surface was provided by coating the boards 



TIME AND 

FREQUENCY 

INVERSION 


TIME 

INVERSION 


NORMAL 

^SPEECH 

FREQUENCY 

INVERSION 




Figure 13. Inversion of time and frequency scales in spectrograms. If scramble contains inverted ele¬ 
ments, these will appear right side up in mechanically inverted spectrogram. Time scale may be inverted 
by rotating elements 180 degrees. Note position of base lines in examples. 


spectrograph. The latter is delicate in texture 
and its surface is easily stained. In this con¬ 
nection it should be noted that the process of 
enlarging the spectrograms does not appreci¬ 
ably affect the decoding time in the case of 
nonrepeated code systems. There will, of course, 
be an initial delay, but in general, the matching 
time will be controlling. Spectrograms can be 
made, enlarged, and cut up faster than they 
can be matched. If it is found necessary, how¬ 
ever, to use spectrograms for matching pur- 


and also the backs of the elements with ordi¬ 
nary rubber cement. This is also the case in 
Figure 14. This latter example shows a two- 
dimensional scramble. Horizontal strips of rub¬ 
berized Bristol board were provided for match¬ 
ing along the time axis. 

Once a system has been thoroughly diagnosed 
certain numerical properties of the coding proc¬ 
ess will be known. Advantage should be taken 
of this knowledge to supplement and check the 
matching process. Examples are given in Pre- 



































CRYPTOGRAPHIC TOOLS AND METHODS 


51 


liminary Reports No. 10,14,22,2^ and of 
Project C-43. 

Effect of Transients 

The two examples thus far cited of spectro¬ 
gram matching were artificially produced by 
cutting up spectrograms of straight speech, and 
the boundaries are therefore clear and sharp. In 
practice the time and frequency boundaries will 
be obscured by transients. Frequency bound¬ 
aries are filter cutoffs, and they are marred by 
overlap or underlap and by phase distortion. 


embodiment of this improvement in a spectro¬ 
graph has not been accomplished because the 
need was not sufficiently pressing in Project 
C-43. 

The basic idea for avoiding the obscuring 
effects of spillover is to permit the spillover to 
take place in such a way as to be subsequently 
removable. For instance, suppose a sample of 
TDS were recorded on the magnetic tape and 
suppose the spectrograph were equipped with 
a suitable switching arrangement such that 
only every alternate element was reproduced. 



Figure 14. Matching spectrograph patterns of two-dimensional scramble. 


This, however, is not as serious as the transients 
occurring at the time boundaries. There is a 
basic difficulty here, arising from the desire to 
obtain a high degree of frequency resolution, 
which entails the use of a narrow scanning 
filter. The response and decay time of such a 
filter is appreciable in comparison with the ele¬ 
ment length in many scrambling systems. The 
decay time produces the more serious of the two 
effects. It causes energy from a strong element 
to spill over into the adjacent following element 
in the spectrogram. This difficulty is unlikely to 
cause trouble in any application of the spectro¬ 
graph except decoding. Therefore, it is felt that 
means for alleviating this difficulty should be 
recorded here. A small amount of exploratory 
work has been done along these lines, but the 


The spillover from each element would then 
occur in a blank area, and it could subsequently 
be trimmed off, leaving a sharp, clear boundary. 
A second spectrogram could then be made of the 
alternate elements, again trimming off the spill¬ 
over. 

Use of Two Scanning Filters 

A logical extension of this idea, which would 
save some time, would be to have two scanning 
filters and use them alternately by suitable 
switching means. Both the inputs and the out¬ 
puts of the filters would have to be switched, 
and the two switches should be separated by the 
appropriate time delay to take account of the 
transmission time through the filter. 

A third variation of this idea which requires 


LiBKAitY 

IlliWWL RESEARCH LABORATORY 


































52 


UNSCRAMBLING AND DECODING METHODS 


less equipment, is to make one spectrogram in 
the usual manner and then make a second spec¬ 
trogram with the machine running backwards. 
The spillover always occurs into the leading 
edges of the elements in spectrograms. Cutting 
the first spectrogram in the proper places will 
result in clear, sharp right-hand edges on each 
element, but each left-hand edge will be ob¬ 
scured by spillover. Cutting the second spectro¬ 
gram in the proper places will give clear 
left-hand edges on each element. Matches could 
then be made between elements from the nor- 


course, might be aggravated intentionally as 
part of the privacy feature of the system. On 
the whole, however, it looks as though ampli¬ 
tude representation should be an improvement 
in decoding work. 

Matching Variable-Area Patterns 

For some purposes it has been found that 
wave form patterns offer certain advantages 
over spectrograms. They can be made more 
rapidly and they can be played back directly to 
reproduce the original speech. Intrinsically, 



Figure 15. Matching variable-area patterns of nonrepeated code TDS. 


mal and backwards spectrograms, in such a way 
as to utilize the good edges of the elements. 

In other respects it is to be expected that the 
patterns produced by the spectrograph can be 
improved. For instance, studies have been made 
which show that amplitudes can be represented 
in such a way that they can be interpreted 
quantitatively. This is an improvement over the 
rather indefinite shades of gray in the usual 
spectrograms. It would provide another criter¬ 
ion for matching. In some cases, however, this 
might be a handicap. For instance, in TDS sys¬ 
tems the pole pieces are not all of equal effi¬ 
ciency. The amplitudes of adjacent speech 
elements are affected by this change in effi¬ 
ciency and they might not appear to match 
when they really should. This condition, of 


wave form patterns are not as good as spectro¬ 
grams for diagnosing frequency shifts and the 
like. However, they present the time scales more 
graphically and they are not subject to tran¬ 
sients at time discontinuities such as the spill¬ 
over effects previously discussed. 

The particular type of wave form pattern 
found most useful was a variable-area pattern 
similar to the sound track used in moving pic¬ 
tures. Variable-area patterns are more distinc¬ 
tive to the eye than oscillographic traces. They 
form geometric designs that catch the eye and 
facilitate matching. The manner of producing 
and playing back these patterns is described in 
Preliminary Reports No. 1,-^ 7,^^ and 12“^ of 
Project C-43. An example of variable-area pat¬ 
terns in process of matching is shown in Figure 


























































CRYPTOGRAPHIC TOOLS AND METHODS 


53 


15 taken from Preliminary Report No. 26-^ of 
Project C-43. 

Variable-area patterns of this type have been 
found particularly good for decoding TDS sys¬ 
tems, especially repeated-code systems. Ampli¬ 
tudes are clearly represented in these patterns. 
By matching a multiplicity of cycles of a re¬ 
peating-code system simultaneously, it is pos¬ 
sible to take advantage of this amplitude 
representation even though the wave form itself 


band. Changes in the split-band code will then 
have no effect on the wave form of patterns pro¬ 
duced in this manner. 

It was also proposed at one time that the use 
of a whisper or monotone instead of normally 
inflected speech would increase the privacy of 
TDS systems. Again this is true in terms of 
spectrograms, but it was found that variable- 
area patterns could be matched almost as easily 
for whispered speech as for normal speech, and 



Figure 16. Oscillographic traces of Vocoder channel signals. 


might be obscured by other features of the 
privacy system. For instance, the use of split- 
band coding was once proposed to increase the 
privacy of TDS systems. This combination 
would be much more private than plain TDS if 
judged on the basis of matching spectrograms, 
particularly if the split-band codes were rapidly 
switched at intervals not simply related to the 
TDS elements. No difficulty, however, was found 
in matching the variable-area patterns to find 
the TDS code. This is described in Preliminary 
Report No. 19^^ which also describes a scheme 
for nullifying the effect of split-band coding on 
the wave form. This consists of modulating all 
the frequency bands down into one frequency 


with the monotone it was actually easier. This 
is described in Preliminary Report No. of 
Project C-43. 

Another feature of the variable-area patterns 
which might be useful is that the patterns have 
characteristic shapes. Usually they look like a 
series of damped oscillations with the highest 
amplitude at the beginning of each fundamental 
period. This should enable the recognition of 
cases in which speech is transmitted backwards. 
The characteristic periodicity of the patterns 
might also be used to recognize whether a fre¬ 
quency band is in its proper location. 

Toward the end of Project C-43 it came to be 
recognized that there would be considerable ad- 









































54 


UNSCRAMBLING AND DECODING METHODS 


vantage in using a compressor in the produc¬ 
tion of variable-area patterns. This tends to 
bring out low-level sounds. The distortion of the 
wave forms resulting from instantaneous com¬ 
pression is immaterial if they are to be used 
only for matching. This kind of compression, 
however, should be sharply distinguished from 
automatic volume control action. The latter is 
relatively slow acting and it is obvious that in 
TDS systems, for instance, it would alter the 
amplitudes of certain elements in such a way as 
to make matches impossible. 

Matching Oscillograms 

Oscillographic traces can be used instead of 
variable-area patterns, although in general 


permuted at short intervals provides a rather 
difficult privacy system to decode. 

It has been found that compression enhances 
the value of oscillographic traces of this type. 
Without compression the lower amplitudes are 
obscured by the width of the traces. Instan¬ 
taneous compression makes changes in the 
magnitude or direction of the traces apparent 
in the lower level sounds. The patterns shown 
in Figure 16 were produced in this manner. 

Indicator Methods 

In the following methods a visual indication 
is obtained denoting which of several possible 
choices puts the speech elements in their proper 
order. These methods are applicable only to 



Figure 17. Time division scrambling [TDS]. 


there will be a disadvantage. For Vocoder 
privacy systems, however, oscillographic traces 
are required. The signals in Vocoder channels 
are essentially fluctuating d-c signals after they 
are modulated down to their normal frequency 
location. They can best be examined in the 
form of oscillographic traces. Figure 16 shows 
a set of undistorted Vocoder channel signals. 
It will be noted that there is a tendency for 
the amplitudes to vary simultaneously in the 
several tracks. It has been found that if the 
signals in the various channels are permuted, 
even with the sharp edges resulting from arti¬ 
ficially produced scrambles, the number of 
mismatches tends to be about 40 per cent. This 
means that a Vocoder system with its channels 


cases where the possible number of choices is 
not overwhelmingly great. A natural example 
of a visual indication occurs in the illustration 
of TDS in Figure 17. Whenever two originally 
adjacent speech elements remain adjacent in 
the scramble the two elements are not separated 
by a time boundary in the spectrogram. Ele¬ 
ments which do not belong in adjacent positions 
have a boundary resulting from discontinuities 
in the harmonics and from spillover effects. 
The absence of a time boundary can be taken 
as an indication that the two adjacent elements 
belong together. To make use of this effect the 
following procedure is suggested. Record a 
sample of the scramble on a loop of tape. Re¬ 
produce this sample through a TDS machine 




























CRYPTOGRAPHIC TOOLS AND METHODS 


55 


and make a spectrogram, noting any adjacen¬ 
cies which occur. Change the code in the TDS 
machine'and make another spectrogram again 
noting adjacencies. A systematic set of codes 
should be worked out in advance which explore 
all the possible combinations of elements. At 
the end of such a cycle of operations it should 


occur at the boundaries of elements which do 
not belong together. These will generate fre¬ 
quencies higher than the cutoff of the high-pass 
filter and will appear as pulses on the scope. 
The absence of a transient will indicate either 
that the elements belong together or that no 
energy was present. Again a systematic cycle 




Figure 18. Effect of rectification on normal and band-shifted speech. A, straight speech rectified; B, six- 
code split-band scramble; C, effect of rectifying six-code split-band scramble. 


be possible to place a large percentage of the 
elements correctly. This can be applied to non- 
repeated or repeated code TDS. 

A variation of this method, which was sug¬ 
gested but not tried and which should be much 
faster, is as follows: Reproduce the recorded 
sample through a low-pass filter, say 2,500 
cycles. Pass it through a TDS machine and then 
through a high-pass filter with the same cutoff. 
View the output of the high-pass filter on a 
cathode-ray oscilloscope whose sweep is syn¬ 
chronized with the TDS cycle. Transients will 


of codes should place most of the elements cor¬ 
rectly. 

Another example of the indicator method 
is the following: Suppose in a split-band D2 
system six known codes are used in an irregular 
sequence, and it is desired to determine the 
sequence. The following procedure is sug¬ 
gested : Record a sample and reproduce it 
through a decoding machine equipped with one 
of the proper decodes, and make a spectro¬ 
gram. Certain elements in the spectrogram will 
be seen to be normal speech. These elements. 
















56 


UNSCRAMBLING AND DECODING METHODS 


of course, are the ones to which the particular 
code applies. It is much easier to determine 
whether a particular element consists of 
straight or scrambled speech, than to deter¬ 
mine which particular code was used. Repeat 
this procedure with each of the other five 
codes. Each element can thereby be identified 
with a particular code. 

Use of Rectification 

A variation of this procedure, which should 
give more positive results, is as follows: The 



non-linear device 


LP FILTER 

TO INDICATING 
DEVICE OR 


OR DEVICES 


100 ~ 

RELAY 


A 


BASIC METHOD 



B 

APPLICATION TO SPLIT BAND DECODING 

Figure 19. Band-shift detector. 


output of the decoding machine used as above 
is rectified before making the spectrogram. 
Rectifying normal speech does not add inhar¬ 
monic components, whereas rectifying speech 
which contains band shifts results in inhar¬ 
monic components. This is illustrated in Figure 
18. The upper spectrogram shows rectified 
straight speech. This looks perfectly normal 
except that the frequency range is somewhat 
more completely covered with harmonics than 
is the case in normal speech. The second specto- 
gram shows a sequence of split-band scrambles. 
The third spectrogram shows a similar sample 
rectified, with none of the elements decoded. 
Rectifying the undecoded elements results in 
a complete smear in the spectrogram compared 
to the rectified straight speech. Properly de¬ 
coded elements will stand out more clearly 
against the background of rectified scrambled 
speech. 


Another variation of the indicator method 
consists in subjecting the scrambled speech to 
a nonlinear device or devices in such a way as 
to obtain difference tones between the compo¬ 
nents. In normal speech, in which all com¬ 
ponents are harmonically related, there will be 
no difference tone lower than the pitch of the 
voice. In scrambled speech the components are 
not harmonically related and there will be dif¬ 
ference tones lower than the pitch of the 
voice. The output of a 100-cycle low-pass filter 
therefore, can be used to indicate whether a 
band of speech is in its proper frequency loca¬ 
tion or not. This is illustrated in Figure 19A. The 
importance of this method lies in the fact that 
each frequency band can be examined sepa¬ 
rately. It might therefore be used to determine 
for each element in a two-dimensional scramble 
which frequency band it came from. 

Figure 19B shows how each band can be 
lifted out of the scramble and placed in each of 
the five possible positions either straight or in¬ 
verted. The spectrograph might be used to 
speed up the analysis process as illustrated in 
Figure 20. The output of the low-pass filter is 
fed to the marking amplifier. Whenever the 
output of the low-pass filter is zero there will 
be no mark produced. Whenever there is an 
output a mark will be produced. The procedure 
would then be as follows: Set oscillator FI at 
one value and then set oscillator F2 succes¬ 
sively at each of its ten values (or five if 
inversion is not required). Repeat with oscilla¬ 
tor FI at each of its other five values. For each 
of these 50 settings allow the spectrograph 
drum to rotate two or three times with a few 
blank rotations between each setting. The 
traces on the drum will then look something 
like the drawing. The time axis is as usual 
disposed lengthwise. If all the traces in a 
given time interval are blank it is presumed 
that this represents a silent interval. Single 
blank intervals in otherwise continuous marks 
indicate that these settings were the correct 
ones. If none of the marks for a particular 
element are blank the indications are that at 
that particular moment a consonant occurred 
which of course is composed of inharmonic 
components. This system was not actually tried 
in this complete form but enough work was 
















CRYPTOGRAPHIC TOOLS AND METHODS 


57 


done to show that it is possible to make use 
of the presence of inharmonic components in 
some such manner. It appears therefore that 
a substantial fraction of the elements in a two- 
dimensional scramble might be identified as 
to frequency location. 

One other possibility of this type might be 
mentioned. Variable-area patterns of vowel 
sounds have characteristic configurations. 
These configurations depend on their harmonic 
structure, and a disturbance of this structure 



SPEECH ON TAPE 


EROM TO 

1 1 

1 li 

2 1 

2 i: 


RESULTING PATTERN 
CODE SWhTCHING POINTS^ 


I I I I i I I I I I I I I' I I I 


^INDICATES 

SILENT 

INTERVAL 


V 

^IN 


INDICATES 

TRUE 

POSITION 


Figure 20. Adaptation of spectrograph for de¬ 
coding switched split-band scramble. 


should change these patterns in a recognizable 
manner. For instance, if the components are 
inharmonic there will be no periodicity at the 
fundamental pitch rate. It might therefore be 
possible to use variable-area patterns, which 
can be produced much more rapidly than spec¬ 
trograms, as indicators along the lines of the 
above discussion. 


Application to Table 1 

In this section we will examine the applica¬ 
tion of cryptographic methods to the specific 
scrambling systems listed in Table 1. In this 
table the systems which might require crypto¬ 


graphic attack are indicated. The following 
paragraph numbers refer to privacy systems in 
Table 1. 

A4. Among the systems listed under single 
modulation the only one that might require 
cryptographic treatment is the phase-reversal 
system. This system is a special case of the 
multiplication system which will be treated 
later. 

B4, C2. Among the double and triple mod¬ 
ulation systems, the irregular continuous 
displacements were not handled by noncrypto¬ 
graphic methods. It might be necessary to make 
a continuous series of spectrograms to deter¬ 
mine the displacements as a function of time. 
This might someday be done continuously and 
instantaneously, in which case compensating 
frequency changes might be made continuously 
by hand to decode the material. 

Dl, D2. Among the band-splitting systems, 
the fixed or slowly switched codes can be solved 
by inspection. If the code is rapidly switched, 
however, single elements seldom contain suffi¬ 
cient information to determine the codes. If 
the switching sequence is a repeated sequence, 
it may be worthwhile for the sake of quality to 
determine the sequence and get in step with it. 
In this case the methods described under the 
heading ‘‘Indicator Methods’’ should be of 
assistance. If the switching sequence is never 



Figure 21. Repeated code multiplication system. 

repeated the indicated noncryptographic meth¬ 
ods appear most reasonable. 

F2, F3. TDS systems yield very poorly to 
noncryptographic attack. For repeated-code 
systems, however, the code can readily be de¬ 
termined by matching either spectrograms or 
































58 


UNSCRAMBLING AND DECODING METHODS 


variable-area patterns, taking advantage of the 
numerical properties of the codes. These meth¬ 
ods are covered in Preliminary Report No. 14^^ 
of Project C-43. Nonrepeated-code systems, 
however, have thus far been found exceedingly 
difficult to handle, although the methods of 
spectrogram matching, matching variable-area 
patterns, and indicator methods apply. Efforts 
in this direction are described in Preliminary 
Report No. 26“^ of Project C-43. 

F4. Speed variations, according to some pre¬ 
liminary laboratory tests, are rather ineffective 
in masking the intelligence of speech unless 
the variations are exceedingly wide and rapid. 
Technical difficulties then become so great that 
this appears to be an unlikely privacy system 
by itself. Small variations in speed, however, 
might be used to make spectrograms of TDS 
systems more difficult to match. In this case, 
however, it will be unnecessary to determine 
the speed variation program if the TDS 
scramble can be removed. 

Gl, G2, G3. Combinations of TDS and fre¬ 
quency scrambles are interesting from the 
cryptographic standpoint. Since repeated-code 
TDS systems were found easy to break, it was 
proposed to add various forms of split-band 
scrambles. It was argued that the continuously 
changing frequency scrambles would alter the 
shapes of variable-area patterns so that 
they could not be matched. Furthermore the 
changing frequency scrambles would make 
spectrograms unsuitable for matching, espe¬ 
cially if the split-band codes were switched 
nonsynchronously compared with the TDS 
boundaries. Each time the frequency code was 
switched a new vertical boundary would appear 
in the spectrogram, and in combination with 
the TDS boundaries the spectrograms would 
be very severely broken up in the time scale. 
It was found, however, as discussed in Pre¬ 
liminary Report No. 19^’^ of Project C-43, that 
if the TDS code is a repeated code the fre¬ 
quency scrambles can be practically ignored in 
matching variable-area patterns. Having found 
and removed the TDS code the remaining fre¬ 
quency scramble can be solved by noncrypto¬ 
graphic methods. 

In the case of nonrepeated TDS, however, 
the addition of split-band coding would increase 


the difficulty considerably, provided that the 
two coding systems do not provide clues to each 
other. The most promising method for handling 
this system appears to be to determine the split- 
band codes first by the indicator methods 
previously discussed. If the split-band codes are 
then removed the remaining scramble can be 
handled as straight TDS. Another possible 
method is to make variable-area patterns with 
all the decodes superposed. The resulting pat¬ 
terns, however, will not be as satisfactory for 
matching as patterns of straight speech. 

G4. The two-dimensional scramble can be 
handled by matching spectrograms if a re¬ 
peated code is used. Experiments along these 
lines are described in Preliminary Report No. 
22-0 of Project C-43. If the code is nonrepeated, 
however, it would be exceedingly difficult and 
time consuming to handle by unaided match¬ 
ing. It would help considerably if the original 
frequency location of each element in the 
scramble could be determined. This might be 
accomplished by the methods described under 
the heading “Indicator Methods.’' 

HI. Determining the code for multiplication 
or phase-reversal systems can be accomplished 
quite readily if the code is repeated at suffi¬ 
ciently short intervals. In the one system which 
was met in Project C-43 (Preliminary Report 
No. 18-^) the code wave was repeated 100 times 
per second. In this case the scrambled signal 
could be applied to the vertical plates of an 
oscilloscope with the horizontal sweep syn¬ 
chronized with the code cycle. It is obvious that 
every time the coding wave passes through 
zero the scrambled signal also passes through 
zero regardless of the value of speech signal 
at the moment. If several cycles of scrambled 
speech material are superposed, therefore, they 
have the appearance shown in the photograph. 
Figure 21. The superposed traces show a defi¬ 
nite pattern, with regions of high and low 
amplitude, and also sharp indentations. These 
latter are the crossover points of the code wave. 
There is also a marked tendency for the peaks 
to occur alternately above and below the center 
line, but the amplitudes of the peaks are not all 
alike. Since the speech amplitudes tend to aver¬ 
age out over a number of cycles, the amplitudes 
of the superposed peaks reflect pretty accu- 



CRYPTOGRAPHIC TOOLS AND METHODS 


59 


rately the amplitudes of the coding wave at 
those points. The probable shape of the coding 
wave based on this evidence, has been partly 
traced in. 

It has been found experimentally that if 
only the crossovers of the coding wave are 
reproduced the speech will be intelligibly de¬ 
coded. The decoding wave need not be the 
reciprocal of the coding wave. It can be like 
the one drawn in at the right in the photograph. 
It is only necessary, therefore, to generate a 
wave having its crossovers at the indicated 
points, and reverse the phase of the scrambled 
signal of these points. 

H2, H3. Level modulations by themselves are 
not private, but they might very well be used in 
combination with other systems in an attempt 
to foil the matching of speech patterns. The 
level modulations themselves, however, need 
not be solved cryptographically. 

Jl. There appears to be no method either 
cryptographic or noncryptographic for break¬ 
ing the noise-masking method if the noise is 
predistorted, random, and sufficiently high in 
level to really mask the speech. These require¬ 
ments, however, make the technical difficulties 
for system operation very great and it is un¬ 
likely that this method can be used over radio 
channels. Cracking this system therefore be¬ 
comes a matter of solving the noise-distorting 
system. Project C-43 had no experience along 
these lines. 

Kl, K2, K3, K4. Scrambled Vocoder channels 
can theoretically be solved by matching oscillo¬ 
grams. Actually as mentioned under the head¬ 
ing “Matching Oscillograms” this procedure is 
very difficult because the channels look so much 
alike. 

LI, L2, L3. Channel-mixing systems would 
be exceedingly difficult to handle crypto¬ 
graphically if a sufficient number of channels 
were involved so that noncryptographic meth¬ 
ods were inapplicable. The only possible method 
of attack appears to be matching spectrograms. 
Since, however, about 25 per cent of normal 
speech consists of pauses, many of the switch 
points will occur in these pauses and it will 
therefore be difficult to establish continuity 
by matching. 


Determination of the Message 

The objective of decoding work is usually not 
to determine the codes used, but to learn the 
intelligence which was transmitted under these 
codes. In the case of repeated-code systems, 
the procedure for obtaining intelligence is 
obvious once the code has been determined 
by the methods outlined above. It is only neces¬ 
sary to set this code into a machine similar 
to that used at the receiving end of the system 
being monitored, and listen directly to the 
transmitted speech. If the material has been 
recorded while the code was being determined, 
the recorded material can in general be de¬ 
coded in the same way. 

In the case of nonrepeated-code systems, the 
determination of the code sequence leaves us in 
general a long way from the determination of 
the message. All the material must first be 
recorded in scrambled form. It is necessary 
during this process to establish time reference 
points in the scramble, perhaps by superposing 
clicks or spurts of tone during the recording 
process, and referring the code sequences to 
these points. A decoding machine must be 
available, such as the one described in Pre¬ 
liminary Report No. 15^® of Project C-43, which 
is adaptable to a variety of coding systems. 
The code sequence must be set into this ma¬ 
chine, perhaps in the form of a punched tape. 
The scrambled material must then be repro¬ 
duced and fed into the machine, maintaining 
proper synchronism between the reproducing 
and decoding systems. This is a formidable job. 

There are some alternative possibilities 
which may apply in special cases. In the case 
of nonrepeated-code TDS, for instance, the 
process of matching variable-area patterns has 
actually restored the speech in reproducible 
form. Variable-area patterns can be played 
back just like the sound tracks used with 
motion pictures. 

Use of Playback 

A playback machine of this type is described 
in Preliminary Report No. 12-^ of Project C-43. 
The rearranged elements are mounted on a 
strip of adhesive, and scanned with a light slit 
and photocell. Considerable noise is caused by 




60 


UNSCRAMBLING AND DECODING METHODS 



—RECORDER UNIT 


I— AMPLIFIER ANALYZER UNIT 


Figure 22. Sound spectrograph (D-165529) on a “push-around.” 


‘—RECTIFIER UNIT 






































THE SOUND SPECTROGRAPH 


61 


the joints between the separate elements, but 
this could be largely eliminated by a specially 
designed squelch circuit, perhaps controlled by 
a separate light beam and photocell to cut off 
the output wherever a joint is passing under 
the scanning beam. The first attempt to use 
this decoding method was unsuccessful, as dis¬ 
cussed in Preliminary Report No. 26-^ of Proj¬ 
ect C-43. However, there is nothing basically 
wrong with the method; it simply needs better 
execution than it received in the first attempt. 

If the solution of the coding system requires 
spectrograms rather than variable-area pat¬ 
terns, it is still theoretically possible to play 
back the rearranged pieces. A playback ma¬ 
chine for spectrograms is described in Pre¬ 
liminary Report No. 17“^ of Project C-43. This 
first model requires a negative transparency 
of the spectrograms, to be scanned by a light 
slit and photocell, with a multifrequency light 
chopper interposed ahead of the photocell. 
Again the method is basically sound. The ex¬ 
perimental machine described in the report 
needs considerable improvement before it will 
yield adequate quality for the purpose described 
above, in order to overcome the degradation 
of quality caused by the joints, by slight mis¬ 
placements of the elements, by spillover at 
the boundaries, etc. Furthermore, to get good 
patterns for matching, the signal must be 
subjected to a very high degree of compression, 
which distorts both the time and the frequency 
distribution of energy. It may be necessary 
to make one kind of pattern for matching, 
and another kind for playback, as was done 
with the variable-area patterns described in 
Preliminary Report No. 26-^ of Project C-43. 

As a final alternative, it is possible to learn 
to read speech spectrograms by visual inspec¬ 
tion. Theoretically, therefore, the rearranged 
spectrograms might yield the message directly. 
Here again, however, the boundary distortion 
will increase the difficulty of reading the pat¬ 
terns. It has also been found that the best 
patterns for matching are not the best for 
reading, and it may be necessary to make two 
sets of patterns. However, since spectrograms 
have been continually improving, the possibility 
of visually determining the intelligence from 
rearranged spectrograms must be listed as a 


distinct possibility, and one which, if it is 
feasible, is the most general of all methods 
since the basic procedure is the same for all 
the scrambling methods which can be handled 
in this manner. 


45 the sound spectrograph 

In the material to follow describing the sound 
spectrograph developed in Project C-32 and 
used continuously in subsequent decoding and 
evaluating projects, a brief and general 
description will be followed by a more detailed 
analysis of this important visual aid to the 
study of privacy systems. The sound spectro¬ 
graph analyzes speech (or other sounds) in 
terms of its three basic dimensions of time, 
frequency, and amplitude. Such analyses, 
shown visually on a graph or chart, are helpful 
in understanding the complexities of sound and 
what various scrambling methods do to speech 
to make it unintelligible. 

In March 1941 an early laboratory model of 
the sound spectrograph was demonstrated as 
an instrument that with further development 
might be useful in studies of telephone privacy. 
It was appreciated at that time that the need 
might arise for intercepting communications 
in scrambled speech and decoding them. It was 
also appreciated that new scrambling systems 
might be encountered and that means would 
be needed for diagnosing such systems. For 
such a purpose the unaided ear has very limited 
capabilities. Such things as oscillograms, which 
show the wave form, provide few clues as to 
the mechanism by which the wave form was 
changed. Project C-32, the forerunner of Proj¬ 
ect C-43, was organized in the fall of 1941 to 
produce a sound spectrograph useful for 
diagnosing and decoding speech scrambling 
systems. 

About a month before the attack on Pearl 
Harbor, patterns that could be used for de¬ 
coding work were being produced with a bread¬ 
board model, and the first finished model of the 
spectrograph was available by the end of that 
year. Additional models of the spectrograph 
were built for the Armed Services, incorporat¬ 
ing improvements in operation and in rugged- 








62 


UNSCRAMBLING AND DECODING METHODS 


ness. The model, described in the final report 
of the Project C-43 and shown in Figure 22, 
has been used in studies of various privacy 
systems submitted by the Army, Navy, and 
NDRC for the purpose of evaluating the degree 
of security which they afforded. Improvements 
in the form of a calibrating circuit built into 
the spectrograph and control circuits added in 
the form of an applique unit, were made as the 
work progressed. 


^ How the Sound Spectrograph Works 

A schematic diagram of the sound spectro¬ 
graph is shown in Figure 23. The signal to be 
analyzed is recorded on a loop of magnetic tape 
at a speed of 25 rpm permitting a sample 2.4 



Figure 23. Schematic diagram of sound spectro¬ 
graph. 


sec long to be recorded. The recorded material 
is then reproduced at 78 rpm. Because of this 
speedup, the original signals which may have 
filled the frequency region between zero and 
3.5 kc now extend to about 11 kc. The signal 
is modulated with a carrier which gradually 
changes in frequency from 23 to 12 kc as the 
recorded material is reproduced repeatedly. 
The lower sideband of the resulting signal is 
passed through a band-pass filter with a center 
frequency of about 12 kc. 

The output of the filter is amplified and fed 
to a stylus bearing on facsimile paper, making 
a trace varying in density with the instan¬ 
taneous energy passed by the filter. The paper 
is mounted on a drum which is geared to the 
turntable rotating the magnetic tape. As the 
frequency of the modulating carrier changes, 
the stylus moves along the drum laterally. The 
resulting spectrogram is built up line by line. 
In this manner a pattern is produced which 


shows by its light and dark areas how the 
intensity in the signal varies as a function of 
time and frequency. 

The change in frequency produced by modu¬ 
lating the voice signal with a varying carrier 
signal of suitable high frequency would not be 
necessary if it were possible to make a band¬ 
pass filter whose center frequency could be 
shifted easily. In this case the actual voice fre¬ 
quencies could be scanned by the filter to de¬ 
termine the characteristics in frequency with 
time. It is easier, however, to accomplish the 
same object by the method actually used in the 
spectrograph. 


Operation 

It is the fact that both time and frequency 
variations are simultaneously displayed which 
makes spectrograms so valuable for decoding 
work. 

Scanning filters of various widths can be used 
for different purposes. If the filter is wide, it 
will give an analysis which is limited in the 
amount of detail it can portray in the frequency 
dimensions, but it will respond quickly to 
changes in amplitude with time, and will there¬ 
fore give sharp time resolution. The narrower 
the filter the more frequency detail is shown in 
the spectrograms, at the sacrifice, however, of 
some of the time resolution. With all the filters 
thus far used, the shift in frequency range 
from line to line is only a fraction of the width 
of the filter. Successive lines in the spectro¬ 
gram, therefore, do not represent successive 
frequency bands. They represent frequency 
ranges which overlap by a large fraction of 
their total width. The density of the patterns, 
therefore, changes very gradually along the 
frequency dimension. 

The kind of patterns produced by this method 
of analysis is illustrated in Figure 24. The 
upper spectrogram in the figure was made with 
a scanning filter about 300 cycles in width. The 
separate words can be plainly distinguished. 
The vowels are distinguished by dark bands 
with vertical striations. The consonants are in 
general less intense and show a different type 
of structure. It will be noted that the dark 

































THE SOUND SPECTROGRAPH 


63 


bands are different in the different vowels, and 
they change not only from one word to the next 
but also within each word. Analyses of this 
type, therefore, graphically portray changes in 
the energy frequency distribution of a complex 
signal with both time and frequency. It should 
be emphasized, however, that the relative in¬ 
tensities of the various components of this 
particular sample of speech, notably the con¬ 
sonants, differ to a far greater extent than 
would be judged by the relative blackness of 
their patterns. In other words, a very large 


discrete harmonics causes the vertical stria- 
tions in the patterns made with the wider filter. 
Whenever the filter is wide enough to pass 
several harmonics at once, these harmonics 
beat with each other and form maxima and 
minima in the output of the filter. The fre¬ 
quency of the beats corresponds exactly to the 
frequency of the voice pitch. 

It will be noted in the 45-cycle spectrogram 
that the harmonics rise and fall in frequency 
from moment to moment. This reflects the 
changing pitch of the voice known as inflection. 



INTERVALS 


Figure 24. Spectrograms of normal speech, words being ‘‘one, two, three, four, five, six. In this case, 
spectrograms are somewhat smaller than normal size due to photographic reduction and some trimming 
at ends. 


amount of level compression is incorporated 
in these patterns. 

The lower spectrogram in the figure shows 
the same words analyzed with a filter only 45 
cycles wide. This filter is narrow enough to 
resolve the individual harmonics of which 
vowel sounds are composed. The harmonics 
consist of the fundamental voice pitch together 
with both odd and even multiples of this fre¬ 
quency. Some of the harmonics are stronger 
than the others, because they are reinforced 
by resonance in the oral cavities as the words 
are formed. It will be noted that the dark 
areas in these patterns correspond in frequency 
and in trend with those in the upper spectro¬ 
gram. The fact that vowel sounds consist of 


Inflection is normally used in connected speech, 
and this fact is of assistance in decoding work, 
because the spacing and trend of the individual 
harmonics in spectrograms provide important 
clues in diagnosing privacy systems. 


Level Compression 

In normal speech there is a tremendous 
change in level from moment to moment par¬ 
ticularly in the level of consonants as compared 
to vowels. There is also a considerable differ¬ 
ence in the average level at low frequencies 
as compared to high frequencies. This latter 
difference can be corrected by predistortion. 



















64 


UNSCRAMBLING AND DECODING METHODS 


and present models of the spectrograph contain 
shaping networks for this purpose. There are, 
however, changes from moment to moment in 
the relative levels of high and low frequencies 
in different speech sounds which cannot be 
corrected by shaping networks. The facsimile 
paper on which spectrograms are made has a 
range of between 10 and 15 db. The range of 
levels in speech greatly exceeds this value. This 
means that if the average level is adjusted 
so that the highest components appear at maxi¬ 
mum blackness, the lowest level components 
will be invisible. Conversely if the level is so 
adjusted that the low-level components appear 
in the pattern, the high-level components will 
severely overload the recording paper. To show 
both the high- and low-level components occur¬ 
ring in speech, therefore, it is necessary to com¬ 
press the instantaneous signal into a much 
narrower range. 

In the earliest models of the spectrograph the 
marking amplifier shown in Figure 23 was 
given a compressing action by means of a 
thyrite varistor across the grid of the output 
stage. Whenever the output of the scanning 
filter was low the gain of the amplifier was 
effectively raised from an average condition 
and whenever the output was high the gain 
was effectively lowered. This tended to equalize 
changes in level with both frequency and time. 
The compressor was replaced by devices which 
can exercise certain types of discrimination in 
controlling the instantaneous gain of the mark¬ 
ing amplifier. These devices are known as con¬ 
trol circuits. They provide patterns with better 
resolution in both time and frequency than 
can be obtained with the compressor. The pat¬ 
terns shown in Figure 24 were made with these 
control circuits in operation. The circuits are 
described in Preliminary Report No. 21 ^^ of 
Project C-43. 


Possible Improvements 

The spectrograph patterns underwent con¬ 
tinual improvement in the course of this work, 
but probably they can be still further improved. 
The control circuits thus far produced are by 
no means the final word. Circuits of this type 


can be adapted to affect the patterns in various 
ways, and it is conceivable that different con¬ 
trol circuits could be developed for decoding 
different types of scrambles. 

One definite line of improvement concerns 
the time resolution. Many scrambling methods 
produce sharp discontinuities of the scrambled 
speech in the time dimension. The process of 
analyzing the scrambled signal in such a way 
as to obtain high frequency resolution tends 
to obscure the signal at these sharp boundaries. 
This is a basic situation which affects not only 
the spectrograph, but also all types of ana¬ 
lyzers. To obtain a high degree of frequency 
resolution, a narrow filter must be used. The 
narrower the filter, however, the longer its 
response and decay time, that is, the output of 
the filter cannot be made to change as rapidly 
in level as the instantaneous level of the signal. 
This causes strong components to spill over 
across the time boundaries. In general this 
spillover does not interfere greatly with the 
recognition of various privacy systems, but it 
does interfere severely where spectrograms are 
to be used for decoding work. Several possible 
remedies for this situation have been devised 
and are recorded in Chapter VI of Part I of the 
final report of Project C-43. 


^ Amplitude Representation 

In the patterns thus far discussed the in¬ 
stantaneous intensity of the signal is repre¬ 
sented by the lightness or darkness of the trace 
in the spectrograms. This representation is 
inherently nonlinear and practically impossible 
to make quantitative. For some types of work 
it would be highly desirable if the amplitudes 
could be represented in such a way that they 
could be interpreted quantitatively. 

Figure 25 shows a spectrogram which upon 
close inspection will be seen to be made up of 
discrete dots. The dots are close together in 
the dark portions of the spectrogram and 
farther apart in the light portions. The dots 
themselves are all of equal blackness. The 
spacing of the dots is in fact quantitatively 
related to the instantaneous level of the signal. 
The level at any point in the spectrogram can, 



THE SOUND SPECTROGRAPH 


65 


therefore, be measured by measuring the dot 
spacing with suitable equipment and comparing 
it with a scale showing dot spacing vs level. 

Another type of representation is shown in 
Figure 26. Here the levels are represented by 
the type of technique used in representing 
topographical variations in contour maps. The 
contour lines each represent regions in which 
the signal reaches a particular fixed level. The 
lines may be spaced so as to represent steps 
of any desired number of decibels, or any 
number of volts. In the lower spectrogram the 


features designed for the specific purpose in 
mind. 


Spectrograph Details 

As described in general terms above, the 
output of the scanning filter of the sound 
spectrograph is recorded continuously on fac¬ 
simile paper wrapped around a drum rotating 
with the magnetic tape so that one revolution 
of the tape corresponds to one rotation of the 



Figure 25. Method of representing amplitudes in such a way that they can be interpreted quantitatively 
by use of discrete dots all equally black. Dots are closely spaced in dark regions and widely spaced in 
light regions. There is definite quantitative relation between dot spacings at any point and level of 
signal at that point. Level could, therefore, be determined by measuring dot spacing. 


spaces between the contour lines have been 
filled in with various densities of dot spacing. 
This permits instant recognition of equality 
of level in different portions of the signal. 

Quantitative amplitude representation may 
or may not prove useful in decoding work. For 
certain kinds of signal it should prove useful, 
because it provides another dimension besides 
time and frequency which can be used for 
determining continuity or discontinuity in the 
signal. In other cases, however, it may prove 
useless, because changes in level have arbi¬ 
trarily been introduced into the scramble. 

The developments mentioned above empha¬ 
size the fact that the sound spectrograph is a 
highly flexible device and its capabilities along 
any line can be greatly increased by adding 


drum. The drum is moved laterally by a lead 
screw as the modulating frequency is slowly 
changed. 

The analyzer circuit comprises a variable- 
frequency oscillator, a balanced modulator in 
which the output of the oscillator is mixed with 
the voice frequencies, and the scanning filter. 
The balanced modulator automatically prevents 
any signals from getting into its output circuit 
except the resultant sidebands produced by the 
modulating or mixing process. Thus neither 
the original voice frequencies nor the oscillator 
frequencies appear in the output. Since the 
band-pass scanning filter has a mid-band fre¬ 
quency of about 12 kc it automatically selects 
or passes the lower sideband produced by the 
modulating process. The sidebands appearing 















66 


UNSCRAMBLING AND DECODING METHODS 


in the output of the modulator have energy- 
frequency distributions identical to the energy 
of the modulating signal and occupy a position 
in the frequency scale corresponding to the 
carrier frequency. A change in the carrier or 
modulated signal of, for example, —200 cycles, 
will cause the two modulator output sidebands 
to shift to a position in the frequency scale 
200 cycles lower. 

The scanning filter has a mid-band frequency 


subsequent extensions of this project, the spec¬ 
trograms are slightly over 12 in. long normally 
representing a recording of 2.4 sec making the 
time scale approximately 200 msec to the inch. 
The records are normally 2 in. wide covering 
3.5 kc making the frequency scale about 1/16 
in. per kilocycle. The frequency dimension can 
be expanded if desired but this requires that 
a longer time be available for making the 
record. 



Figure 26. Amplitude representation by contours, every point on any one line representing equal signal 
level, and successive lines starting from blank background representing successively higher levels. In 
upper spectrogram, it is not immediately apparent which regions are peaks and which are valleys. In 
lower spectrogram, areas between successive lines have been filled in with patterns made up of discrete 
dots. Closer dots indicate higher level, with regions of equal level in different parts having same dot 
spacing. 


such that when the carrier has one extreme 
value, only the lowest frequency components of 
the lower sideband will fall within the pass- 
band of the filter. When the carrier has its 
other extreme value, only the highest frequency 
components of the same sideband will pass 
through the filter. As the carrier frequency 
is slowly changed, all frequencies of one side¬ 
band (in this case the lower sideband) will be 
scanned by the filter and the relative strengths 
of the signals from moment to moment will be 
impressed upon the recording facsimile paper. 

The record is made on what is known as 
Teledeltos paper whose light-colored surface is 
blackened by the passage of an electric current 
from the metal stylus through the paper to 
the metal drum on which it is wrapped. 

In the model employed in Project C-43 and 


Applications of the Spectrograph 

Although the spectrograph was developed for 
use in analyzing speech privacy systems and in 
decoding scrambled speech records, it has more 
general application. The spectrograms shown 
here are characteristic of what the instrument 
can do in analysis and illustrate the results 
obtained with different filters, different scan¬ 
ning rates, and different types of material. The 
illustrations (taken from the October 1, 1943 
reporU^ on Project C-43) are for the most part 
familiar sounds selected to permit a mental 
comparison of the sound and its time-frequency 
pattern. These illustrations are followed by ex¬ 
amples of the effects of various scrambles upon 
speech. 

Perhaps the most familiar example of a wave 












THE SOUND SPECTROGRAPH 


67 


which is complex in both the frequency and 
time dimensions is speech, which therefore 
provides excellent material for illustrating 
what the spectrograph can do. Figure 27 shows 
a spectrogram of the sentence, “We shall win 
or we shall die,” spoken in a normal manner by 
a male voice and scanned with the narrow 
filter. The time and frequency axes are indi¬ 


lines, they are the separate harmonics of the 
voice pitch, flowing up and down as the voice 
is inflected. The unstriated sounds are unvoiced, 
such as the “sh” in “shall.” 

Figure 28 shows spectrograms of the same 
sentence made with three different filters. In 
the one made with the widest filter, the separate 
harmonics are no longer visible, but the areas 




Figure 27. Spectrogram of “We shall win or we shall die,” with “gray scale” below, showing intensity 
variation relation. 


cated by appropriate scales. The intensity 
variations are indicated by variations in shade 
as shown in the scale below the spectrogram. 
It should be emphasized that there are about 
400 horizontal scanning lines in this 4-in. 
spectrogram; too close together to be seen 
individually. The horizontal striations which do 
appear in the spectrogram are not scanning 


of resonance are brought out more clearly. 
Figure 29 shows a comparison between normal 
speech, a monotone, and a whisper, all by the 
same voice. In the monotone the harmonics 
are straight horizontal lines, and in the whisper 
the harmonics disappear almost entirely, but 
the dense regions by which the different sounds 
are recognized still persist. 













68 


UNSCRAMBLING AND DECODING METHODS 


Figures 30 and 31 show some musical effects. 
The soprano solo shows how the vibrato affects 
both pitch and intensity. The harmonics, it will 
be noted, are much farther apart than in the 
male voice. In the piano music, the notes show 
tapering traces, as would be expected from 
their nature. The other illustrations require 
no special comment, except to note that the 


kc. The lowest spectrogram shows an accelera¬ 
tion measurement. It was desired to find how 
long it took a phonograph record to come to 
full speed after being released, the turntable 
running at full speed all the time. This was 
accomplished by simply recording an 8-kc tone 
on a sample record, and capturing the output 
of the reproducer during acceleration. The time 



Figure 28. Same sentence as shown in Figure 27 recorded through filters of different bandwidths. A with 
45-cycle filter; B, with 90-cycle filter; C, with 300-cycle filter. 


telephone bell was analyzed with the wide filter 
to show the time pattern more clearly. It may 
be seen that the clapper hits two bells alter¬ 
nately, but somewhat irregularly. 

Figure 32 shows some applications in which 
the spectrograph provides a convenient method 
of obtaining a graph of frequency versus time. 
The upper spectrogram shows the output of a 
particular slowly warbling oscillator, and the 
second shows a more rapid sawtooth sweep 
frequency, the latter spectrogram covering 11 


consumed in acceleration, 130 msec in this case, 
is directly indicated. 

Figure 33 shows some more 11-kc spectro¬ 
grams, illustrating the fact that some common 
sounds cover a very wide frequency range. 
Figures 34 and 35 return to the 3.5-kc range 
to show longer samples. They require no special 
comment. 

Contrasting with these illustrations. Figure 
36 shows an example of a steady wave, namely 
thermal noise. Two different levels are shown, 













THE SOUND SPECTROGRAPH 


69 


one of them analyzed with both the “narrow^^ 
and the '‘wide’" filter. It will be noted that some 
of the sounds in the previously discussed spec¬ 
trograms have components which are of the 
same nature as thermal noise, while some have 
components of definite frequencies. The latter 
can be recognized by their solid texture in the 
spectrograms as compared to the characteristic 


well as complex frequency structure because 
the ability to show both at once is the unique 
feature of the spectrograph. 

Speech Privacy Problems 

Even in a steady flow of speech, the distri¬ 
bution of energy over the frequency range is 
constantly changing. Voiced sounds have a 



Figure 29. Spectrograms showing comparison of: A, normal speech; B, monotone; C, whisper. 


texture of a random wave form exhibited by 
thermal noise. The “sh” in “shall,” and the 
roar of the flame, are examples of random 
sounds, while components of deflnite frequency 
may be seen in the jingling of keys (Figure 33) 
and the filing of metal. 

These illustrations, which were chosen for 
variety and for interesting features, show what 
the spectrograph will do. Most of them, as 
emphasized before, show time variations as 


definite structure, consisting of a series of har¬ 
monics of the fundamental voice pitch, the 
harmonics being stronger in some frequency 
regions than in others. Unvoiced sounds have 
no such definite structure, but show a “smear” 
of energy which may or may not be concen¬ 
trated in definite frequency regions. Words and 
sounds are recognized by their energy pattern 
in both time and frequency. Different speakers 
uttering the same sentence will produce pat- 



























70 


UNSCRAMBLING AND DECODING METHODS 


terns which show a distinct general resem¬ 
blance, but also marked differences. The speech 
pattern also may be considerably distorted by 
artificial means before the ear fails to recognize 
the speech, provided the distortion is not too 
discontinuous in frequency or time. Since 
privacy systems depend for their effectiveness 
on distorting the speech pattern beyond the 
possibility of recognition by the ear, it seems 


speech than the low-frequency regions. In these 
patterns, as before, the horizontal scale is time 
(about 1.8 sec is represented in each example), 
the vertical scale is frequency (the upper limit 
is 3 kc), and the density or blackness represents 
the intensity of the energy in a given region. 
It should be noted that the resolution of the 
process is sufficient to separate each harmonic 
in the voiced sounds. This is important, as will 



Figure 30. Musical effects. A, soprano voice unaccompanied; B, piano music; C, orchestra music. 


reasonable that this distortion would also be 
visible to the eye if the scrambled speech pat¬ 
tern could be reduced to suitable graphic form. 

Examples of such patterns are shown in the 
attached photographs taken from the final 
reporU^ on Project C-32. Figure 37 shows some 
normal speech, undistorted except that a slop¬ 
ing network was introduced in the electrical 
circuit to bring out the high-frequency struc¬ 
ture, since this is always weaker in normal 


be seen later, because normally the voice funda¬ 
mental is constantly changing, that is, the voice 
is inflected, and since the harmonics are mul¬ 
tiples of the fundamental, the higher harmonics 
show progressively more change than the 
fundamental. For instance, if the fundamental 
goes from 100 to 200 cycles the tenth harmonic 
goes from 1,000 to 2,000 cycles, a difference of 
1,000 cycles as compared to 100 cycles for the 
fundamental. The traces of harmonics in the 
































THE SOUND SPECTROGRAPH 


71 


visual speech patterns will, therefore, have 
greater slopes at the high end of the pattern 
than at the low end. This may be seen quite 
clearly in the examples of Figure 37. 

Figure 38 shows patterns of some vowel 
sounds. In making these patterns, an attempt 
was made to enunciate clearly, and also to keep 
the pitch constant (monotone), so as to show 


showing transitions from one vowel to another. 
These pairs were chosen because their time 
characteristics are direct opposites. 

Scrambled Speech Patterns 

Figure 40 shows the output of a privacy sys¬ 
tem which depends on simple inversion. In the 
inverted speech, the slopes of the harmonic 



Figure 31. Sound effects. A, whistling with warble or rising note; B, police whistle; C, telephone bell. 


the difference in energy distribution for these 
sounds. These are of interest because of the 
possibility that the visual patterns themselves 
might give clues as to the words they contain. 
Figure 38 also shows the effect on the frequency 
resolution of widening the band-pass scanning 
filter. The wide filter gives much better reso¬ 
lution in time, however, as will appear subse¬ 
quently. 

Figure 39 shows some diphthong patterns. 


traces become greater towards the bottom of 
the pattern, which is the direct opposite of 
normal speech, and is therefore a definite sign 
of inversion. The pattern also thins out at the 
bottom, but this could be altered by a suitable 
distorting network. No network, however, can 
change the slopes of the harmonic traces. Inci¬ 
dentally, the carrier '‘leak” shows up in the 
pattern, giving a direct indication of the fre¬ 
quency about which the inversion was per- 











72 


UNSCRAMBLING AND DECODING METHODS 




^ C C c ’r ’o o c r j o., o > "> > O' o "O o :» 



Figure 32. Spectrograph applications in which frequency versus time curve is presented. A, output of 
warbling oscillator; B, sawtooth generator output; C, acceleration of phonograph record on moving 
turntable. 






























THE SOUND SPECTROGRAPH 


73 


formed. If the carrier is completely suppressed, 
however, its location may be determined by 
trial. 

A more complicated privacy system is the 
split-band system, used in transatlantic radio¬ 
telephony, in which the frequency range is 
divided by filters into several bands, which are 
then arranged in a different order, and some 
are inverted. Figure 41 shows patterns of the 


Looking at the inflected portions, it is quite 
easy to find one which is either definitely in¬ 
verted or definitely erect. The other bands can 
then be immediately labeled inverted or erect 
depending on whether they have the same 
direction of curvature as the band previously 
identified. Now, the relative slopes of the har¬ 
monic traces in the different bands indicate 
their original position in the frequency scale; 



Figure 33. Spectrographs showing that sounds exist in high-frequency region. These are 11-kc, 1,000- 
cycle filter records. A, sound of crumpling paper; B, sound of tearing cloth; C, jingling keys. 


output of such a system with two different 
codes. Both of these samples contain portions 
in which the voice was quite markedly inflected. 
The fact that the frequency range has been 
divided into five bands is quite apparent from 
discontinuities in the energy distribution, and 
also from discontinuities in the harmonic 
traces. It is also quite apparent that some of 
the bands have been inverted because the voice 
cannot be inflected both up and down at once. 


the band showing the least slope (or curvature) 
must originally have been the lowest band, and 
the band with the greatest slope (or curvature) 
must have been the top band. 

The scrambling methods thus far illustrated 
alter the frequency characteristics of the speech 
patterns. The TDS privacy system, discussed 
in detail in Chapter 2, operates on the time 
characteristics of speech, dividing successive 
sections of speech, each m seconds long, into n 























74 


UNSCRAMBLING AND DECODING METHODS 


short time elements which are sent in a 
scrambled time sequence. Each syllable is cut 
up and received as short bursts of energy in 
the wrong order. The number of scrambled 
orders available increases very rapidly with n. 
Systems have been developed in which m is as 
short as 0.6 sec, and n is 20, making each 
element 30 msec. A pulse of tone is sent every 


known. Means must then be found for deter¬ 
mining the code, and this decoding process 
must be repeated every time the code is 
changed. If the code is changed often enough, 
the decoding will lag far behind the message. 
It is essential, therefore, that every artifice be 
employed to increase the speed of decoding. 

Figure 42 shows some speech patterns scram- 



Figure 34. Common sounds recorded in 3.5-kc range, 300-cycle filter. A, striking match and flame; B, filing 
on metal; C, machinery noise; inset is same with 45-cycle Alter. 


m seconds to keep the transmitter and receiver 
in synchronism. In one such system over 60,000 
codes are available, and they may be changed 
quite readily. 

Decoding the TDS Speech 

Obviously, if it is desired to decode such a 
privacy system, it is necessary first to evaluate 
m and n. Presumably a machine could be built 
to unscramble the speech if the code were 


bled by a TDS system. It is apparent that the 
speech has been chopped up on the time scale 
rather than on the frequency scale. It is easy 
to determine n by the length of the elements, 
and since the synchronizing pulse shows in the 
pattern, the most natural assumption is that 
m is given by the distance between these pulses 
or some multiple of it. In the illustration of 
Figure 42, duplicate patterns were made, each 
element was numbered, and one of the 











THE SOUND SPECTROGRAPH 


75 


scrambled patterns was then cut up and re¬ 
assembled, giving the code. It should be noted 
that in the scrambled patterns a few elements 
within each code cycle immediately stand out 
as probably belonging together, particularly 
when voice inflection occurs. Usually the other 
elements in a section cannot be positively 
matched. It is of tremendous help, therefore, if 


identical patterns are mounted on movable 
slides, and viewed through a system of mirrors 
which superposes the two, but all the upper 
pattern to the right of a definite line is blocked 
out, and all the lower pattern to the left of this 
line is blocked out, so that effectively any two 
elements may be juxtaposed to see whether 
they look as though they were originally con- 




Figure 35. Additional 3.5-kc sounds. A, splashing stream of water; B, air bubbles blown through water; 
C, riffling stack of file cards. 


the scrambling order is repeated over and over. 
A doubtful match can be checked in another 
section, and matches which are impossible to 
spot in one section can be readily spotted in 
another. 

Rather than cut the pattern up as in the 
illustration, an optical system has been built 
for viewing two duplicate patterns simultane¬ 
ously. This is shown in Figure 43. The two 


secutive. If a match is discovered in one section, 
the viewer may be shifted without moving the 
slides, for an immediate check in other sections. 
Instead of dividing the scrambled pattern with 
lines and numbering them as in Figure 42, 
suitable scales and numbers for a particular 
TDS system can be incorporated in the slides. 

Speech patterns have been shown to be useful 
in decoding all systems of speech privacy known 













































































76 


UNSCRAMBLING AND DECODING METHODS 


to be in use. This method, however, which is 
quite general, may not necessarily be the speedi¬ 
est in all cases. 

Special Methods for TDS 

The TDS system appears to be the most diffi¬ 
cult to decode of all the speech privacy systems 
known to have been reduced to practice, par- 


time characteristics have been scrambled, the 
wave form itself provides evidence which can 
be visually interpreted. Various methods have 
been tried, the first being the ordinary oscillo¬ 
graph which discloses that there are discon¬ 
tinuities in time more sudden and frequent than 
occur in normal speech. It is possible to cut up 
such traces and piece them together. Examples 



V c C V P O 0 :0 o. v' 0 0 , 0 V ... 



Figure 36. Thermal noise as example of steady sound. Two upper records made with wide filter, two 
levels, 12-db apart. Lower spectrograph made with narrow filter. These are 11-kc records. 


ticularly if a large number of codes are avail¬ 
able and if they are changed often. A great deal 
of emphasis naturally was placed on TDS in 
Division 13 work. The following sections dis¬ 
cuss methods particularly applicable to TDS, 
investigated in parallel with the speech pattern 
development. 

Wave Form Traces. Speech patterns of the 
type thus far discussed are particularly de¬ 
signed to display the frequency composition of 
speech, so that distortion of the frequency scale 
could be recognized visually. Where only the 


of such records are given in the final reporU^ of 
Project C-32. 

It was thought that a variable-area sound 
track would provide more distinctive patterns 
than oscillographic traces, and would have the 
additional virtue that they could be played 
back, and could, therefore, serve perhaps as the 
primary record of the intercepted message. 
This scheme has proved to be useful. Examples 
of records made in this manner will be found 
in the final report on Project C-32 and in sev¬ 
eral of the preliminary reports of Project C-43. 







THE SOUND SPECTROGRAPH 


77 


Partial Matching. The above methods would 
not serve if the TDS code were changed very 
often, in the extreme case if it were changed 
every cycle. One branch of the investigation 
has, therefore, attacked TDS from a statistical 


national Business Machine punched cards, 
thereby enormously reducing the number of 
codes remaining possible. The most complex 
TDS system under consideration in Project 
C-32 has twenty elements per code cycle with 



our 


took 


WAITING AT 


you CA wr judge a do ok 


Figure 37. Normal undistorted speech with sloping network to bring out high-frequency structure. 


angle. In a system with a sufficient number of 
elements the total number of available codes 
is very large. If, however, in a given code cycle 
a few elements can be visually matched, the 
others being inconclusive, it appears possible 
to tabulate in advance all the codes which will 
satisfy the observed matches, perhaps on Inter- 


over 60,000 good codes available. This may be 
reduced to only eight possible codes by match¬ 
ing two groups of three elements. These eight 
codes might conceivably be tested successively 
by automatic means, the correct one being rec¬ 
ognized by ear. Presumably, the message could 
thus be decoded cycle by cycle. A cycle contain- 














78 


UNSCRAMBLING AND DECODING METHODS 


ing insufficient material for visual matching 
may contain no indispensable portion of the 
message either. 

Further Possibilities 

Assuming that the methods outlined apply to 
all privacy systems which it is desired to crack, 
further work would be directed toward speed¬ 
ing up the processes. Improvements in speed 
can, it appears, be made in all of the processes 


Desirable sections could be stopped at any time. 
This appears, however, to require considerable 
equipment as well as considerable development, 
and would be undertaken only if it appeared 
quite certain that speech patterns afforded the 
best means of keeping up with a rapidly chang¬ 
ing code. 

Large Variable-Area Patterns. The variable- 
area pattern method discussed above did not 
require photographic film, but the patterns had 



Figure 38. Patterns of vowel sounds showing effect of wide and narrow filters. 


outlined previously. These improvements may 
be summarized as follows: 

Instantaneous Speech Patterns. With the 
sound spectrograph the selected sample of 
speech is scanned at more than twice its normal 
speed, the filters, etc., being designed for this 
purpose. It appears quite feasible, by pushing 
this process up into television frequencies, to 
obtain speech patterns of the same type on the 
face of a cathode-ray tube, for instance, repre¬ 
senting either a “still” or a “moving” picture 
of, for example, 1 sec of speech. In the latter 
case the picture would be running off one edge 
of the screen and onto the other continuously. 


to be photographically enlarged for easy in¬ 
spection and handling, which is a slow process 
at best. It does not appear impractical, particu¬ 
larly if it is not necessary to play the record 
back, and if the high frequencies are going to 
be modulated down, to develop a simple record¬ 
ing system to produce patterns of the variable- 
area type big enough to see and handle without 
enlargement, thus providing instantaneous pat¬ 
terns. For cases where the code is repeated, a 
cylinder, revolving synchronously once per 
code, with a corresponding lateral movement, 
would give a spiral record on which the simi¬ 
larly located elements of each code cycle would 









'-prt 


THE SOUND SPECTROGRAPH 


79 




T 

aJL^ 


YAH 

-ee clA^ 


YAW 
SEE auAr" 


OX 

CUuJ"' xiJL 


Figure 39. Diphthong patterns showing transition from one vowel to another. 

















80 


UNSCRAMBLING AND DECODING METHODS 



TN^ERTED 


rf\THER'6 





UNmEHTED 




: TOOK 


FATHER'S 


SHOE BENCH OUT 


Figure 40. Spectrographs showing effect of inverting speech. 











THE SOUND SPECTROGRAPH 


81 



NUMBERS INDICATE ORIGINAL ORDER. 
A, INDICATES INVERTED BAND. 


Figure 41. Patterns from split-band privacy system. 






























82 


UNSCRAMBLING AND DECODING METHODS 




1,3-SCRAMBLED 

2,4“REARRANGED IN ORIGINAL ORDER 
y MARKS SYNCHRONIZING PULSE 


Figure 42. Examples of TDS speech with elements cut apart and reassembled. 





































































THE SOUND SPECTROGRAPH 


83 


be vertically arranged so that several matches 
could be seen simultaneously.^^’ 

Decoding by Automatic Trial. It appears 
quite feasible to combine a modified TDS re¬ 
ceiving machine with a crossbar switch system, 
actuated by punched cards, perhaps, so as to 
try successive codes until one unscrambles the 
speech. This is particularly applicable to cases 
where the code is changed often, but where the 
number of possibilities can be greatly reduced 
by visual means. It is also applicable to cases 
where the total number of available codes is 
small. 


eral ways. First a visual inspection will tell 
whether portions of the speech are inverted or 
not and the discontinuities in frequency or time 
or pattern will reveal the coding system used 
at the remote transmitter. The individual seg¬ 
ments of the spectrograph may be cut apart 
with shears and reassembled so that the un¬ 
scrambled pattern may become evident. Having 
determined the nature of the scramble, equip¬ 
ment can be assembled or existing equipment 
can be adjusted so that future samples of the 
scrambled speech may be translated audibly as 
they come in. 



Figure 43. Optical system for comparing duplicate patterns simultaneously, avoiding necessity of cutting 
up spectrograph. 


Decoding Equipment. Equipment might be 
assembled for actually decoding scrambled 
speech. For instance, a system of adjustable 
filters and carriers might be built to take care 
of all split-band systems. This includes shifting 
and inverting frequency bands, introducing dif¬ 
ferent delay into various bands, removing 
bands of noise, wobbling the carrier, and what¬ 
ever other frequency distortion may be in¬ 
cluded. A TDS decoding system with adjustable 
elements and codes also deserves consideration. 

4.5.8 Additional Material in Project C-32 

In actual use, the spectrograms made of rec¬ 
ords of scrambled speech are analyzed in sev- 


In the final reporU^ on Project C-32'' will 
be found a discussion of methods for making 
speech patterns other than those described 
here, and the results of using certain foreign 
language records (Linguaphone) in an effort 
to discover if the spectrograph was universally 
usable without the necessity of having foreign 
language experts on the decoding staff. It was 
found that the fundamental characteristics of 
speech are universal and that the means by 
which a recorded sample of scrambled speech 
was distorted could be worked out even if those 
in charge of the equipment did not understand 
the language employed. 

a Project C-32, Contract No. OEMsr-230, Western 
Electric Company, Inc. 







84 


UNSCRAMBLING AND DECODING METHODS 


4 6 PRACTICAL EVALUATION OF PRIVACY 
SYSTEMS 

Experience has shown that there is a strong 
tendency to underestimate the security or mili¬ 
tary value of a given privacy system as soon 
as laboratory studies have indicated that the 
system can be cracked. An attempt will be 
made here to point out the great difference 
between what might be termed theoretical or 
laboratory evaluation and practical or field 
evaluation, written from the standpoint, not of 
the man interested in decoding a system, but 
of the man interested in getting a practical 
privacy system into use in the field. 

Cracking Time 

The objective of a laboratory study of a pri¬ 
vacy system is to obtain some kind of quanti¬ 
tative measure of the time or effort required 
to decode the system. The questions are: “How 
long does it take to determine the code, and 
how much equipment and how many people are 
required?” The coding and decoding processes 
are studied in detail, possibly with the aid of 
mathematical analysis, to determine whether 
there are any weaknesses or any characteristics 
of the coding process of which advantage might 
be taken to assist in the cracking process. Pos¬ 
sibly a noncryptographic method will be found 
to apply. In this case the cracking time reduces 
substantially to zero. If noncryptographic meth¬ 
ods are not applicable, available cryptographic 
tools and methods are brought to bear. Usually 
a new scrambling system will require modifica¬ 
tions or changes in the existing tools or tech¬ 
niques. Possibly the basic methods can be im¬ 
proved for use against this particular system, 
or possibly new methods can be devised. Pre¬ 
sumably after all this development work the 
project personnel will have become skilled in 
the art of decoding this particular system. The 
cracking time can then be determined quanti¬ 
tatively, perhaps with estimates as to how far 
this may be reduced by further skill. 

In the case of repeated-code systems, the 
cracking time determined in the above way 
substantially represents the total decoding time, 
because, as mentioned previously, this code can 
be set into a receiving machine and the message 


obtained directly. Some additional time might 
be added, however, for determining what was 
said during the time that the code was being 
determined. 

The procedure outlined above is very well 
illustrated in the series of preliminary reports 
on Project C-43 covering the development of 
cracking methods for the repeated code TDS 
system. They include mathematical analysis,^^- 
the development of a new decoding tool,^^. 23 and 
the reduction of the decoding technique to a 
routine.i^ In the case of the multiplication sys¬ 
tem, the chronological steps are in one report.^^ 

Too often the cracking time, as determined 
above, is quoted without qualification to de¬ 
scribe the security of a system. It is, of course, 
usually understood that the use of this figure 
involves the following assumptions: (1) that 
the enemy knows all about the coding system, 
(2) that he is equipped with an adequate supply 
of the machines (our own models may still be 
far from the production stage), (3) that he 
has developed the same decoding tools and tech¬ 
niques that we have (some of our tools may 
be entirely new and secret), (4) that he is 
equipped with an adequate supply of the de¬ 
coding tools, (5) that he has men trained in 
their use, and (6) that he is in a position to 
receive a good signal free of interference. Such 
assumptions certainly represent an extreme 
possibility. Experience has shown that there is 
a strong tendency to forget just how extreme 
a condition such assumptions represent. Even 
if the assumptions are valid there are still other 
factors which affect the military value of a 
privacy system. 

Nonrepeated Code Systems 

If the code is changed periodically it may be 
necessary to have several decoding teams work¬ 
ing in parallel to keep up with the transmitted 
material. The number of teams which will be 
required depends on the relation between the 
intervals of the code changes and the cracking 
time. No particular difficulty presents itself in 
expressing the decoding effort under these con¬ 
ditions in terms of man-hours. The evaluation 
is complicated, however, by the necessity for 
additional equipment, not only for decoding but 
for recording. 




PRACTICAL EVALUATION OF PRIVACY SYSTEMS 


85 


In the case of nonrepeated code systems, the 
cracking time for any given portion of a mes¬ 
sage will, in general, be long compared to the 
duration of that portion of the message. Every 
portion of the message must be cracked indi¬ 
vidually, and the decoding effort can be ex¬ 
pressed as a ratio of decoding time to message 
time. This ratio may be 1,000 or 100,000 to 1, 
that is, each second of message will take 1,000 
or 100,000 teams to keep up with the messages 
as they are spoken. 

This kind of evaluation is somewhat unsatis¬ 
factory, because the length of time it will take 
the enemy to determine the intelligence in a 
particular sentence which might carry military 
information will depend on whether or not he 
is at the moment working on this sentence or 
whether he is wasting his time decoding previ¬ 
ous material which might contain no informa¬ 
tion of value to him. In fact, it has been pro¬ 
posed that the security of such high-privacy 
systems could be materially enhanced by keep¬ 
ing the circuit 100 per cent busy with all kinds 
of material, possibly even from recordings, 
making certain that the enemy has no way of 
determining when the circuit is being used for 
passing important information. As in the case 
of nonrepeated code systems, it seems a bit 
unrealistic in evaluating such a system to as¬ 
sume that the enemy will seize upon the few 
seconds of message time which are important, 
and to compute the length of time it will take 
him to decode that portion of the message. 

Code Analysis 

Many schemes have been proposed for gen¬ 
erating ever-changing codes by a combination 
of short cycles geared together in such a way 
that the number of elements in the cycle is the 
product of the number of elements in the indi¬ 
vidual cycles. One scheme is to use odd ratios, 
such as 99 to 100, so that the code cycle will 
not repeat until the smaller wheel has made 
100 revolutions. In other words there are 9,900 
steps in the code cycle before it repeats. Another 
scheme is the cyclometer type in which one 
wheel rotates one step for each revolution of 
another wheel. Again the total cycle is the 
product of the number of steps on the indi¬ 
vidual wheels. 


Such schemes should be distinguished from 
truly nonrepeating codes, because wherever 
cyclic processes are used, they are subject to 
analysis. This is a matter pertinent to the field 
of cryptanalysis and will not be discussed here. 
In general, it may be said that the difficulty of 
solving such long cycles is not determined by 
the total length but rather by the length of the 
individual subcycles. 

Systems designed to produce a long code 
sequence usually contain provision for read¬ 
justing or realigning certain elements period¬ 
ically or from day to day. Assuming that we 
know all about the system except the momen¬ 
tary settings, estimates can usually be made of 
the length of time and the number of people 
it would take to determine the unknown set¬ 
tings by analyzing a given sample of the code 
sequence. The analyst requires a knowledge of 
the code for a long sequence of scrambled 
speech before he can begin the work aimed at 
determining the unknown settings. He must 
obtain and solve a sufficiently long sample of 
the scramble and then analyze this sequence 
to obtain the settings. Too frequently the eval¬ 
uation of a coding system is based on the 
analyzing time alone whereas the time required 
for solving or unscrambling a long sequence of 
scrambled speech may be overwhelmingly 
greater than the analyzing time. In fact if there 
is no way of solving the code sequence from 
the scramble alone, then the analyst can con¬ 
tribute nothing, and the system is still secret 
regardless of any inherent weakness of the 
cyclic coding system. 

Field Evaluation 

The continuously changing military situa¬ 
tions of modern warfare require rapid means 
of communication in order that the required 
military actions can be taken. A perfectly secure 
speech privacy system is of no military value if 
it requires so much time for encoding and 
decoding that it slows up the communication 
system to the point where appropriate steps 
cannot be taken when needed. Similarly, a 
cracking system is of no value if it is too slow 
to permit countermeasures to be taken accord¬ 
ing to the intercepted intelligence. For certain 
purposes 15 min or even 5 min cracking time 




86 


UNSCRAMBLING AND DECODING METHODS 


is much too slow. Where this is true, a privacy 
system giving 15 min or 5 min privacy is just 
as good as one with an hour's security. This is 
important because systems affording a few 
minutes of privacy were developed in portable 
form, whereas those providing longer privacy 
were not. 

Consider also the equipment and trained 
personnel required for decoding intercepted 
communications. As a specific example, the 
small TDS unit required about 15 min for de¬ 
coding but it required a van-load of highly 
specialized equipment.^^ Suppose the small port¬ 
able TDS unit were used in many planes and 
tanks and other mobile equipment that required 
some privacy. Suppose also that different codes 
were used within different groups of units and 
that the codes were changed at some reasonable 
interval. Would it be worth the enemy’s while 
to provide enough decoding equipment and 
enough trained personnel to follow these units 
around and decode their messages? If it is not 
worth his while, then units rated as low in 
privacy may provide high-grade privacy under 
such conditions. 

Obviously the foregoing does not apply if the 
units are used to convey messages between the 
higher echelons of command. In such cases 
the messages have a longer term significance 
to the enemy, and he can afford to devote con¬ 
siderable time and equipment to intercepting 
and decoding them. 

Advantage might also be taken of the element 
of surprise. Suppose we suddenly introduce in 
the field a low-grade privacy system in large 
quantities. How long would it take before the 
enemy diagnosed the system, developed a de¬ 
coding method, manufactured receiving sets of 
the proper type and also decoding equipment, 
distributed these where needed and organized 
and trained personnel to use them? Until he 
has done these things the units provide com¬ 
plete secrecy. A different kind of system might 
then be introduced which would again provide 
secrecy for a time. 

It is intended simply to point out that there 
are other considerations in the evaluation of 
privacy systems than the time it takes a highly 
specialized group, such as the personnel of 
Project C-43, to decode the system under the 


ideal conditions of a laboratory. The decoding 
time alone is often quoted, because it is the 
only element which can be described quanti¬ 
tatively. While there is always theoretical 
agreement about the existence of the other 
considerations, they cannot be pointed out too 
often or too strongly. 


4 7 DIAGNOSIS OF UNKNOWN SYSTEMS 

Before discussing the diagnosis of speech 
privacy systems it should be pointed out that 
facts concerning the origin of unknown signals 
are often very necessary to their correct in¬ 
terpretation. Such things as the frequency, 
strength, and direction of the signals, the loca¬ 
tion and type of receiver, and the manner in 
which the signals were recorded, can be very 
important data. That is why interceptors should 
be equipped with complete knowledge of the 
various kinds of radio systems and transmis¬ 
sions used by both sides, including jamming 
and radar signals as well as telegraph and fac¬ 
simile signals. Some of these signals, particu¬ 
larly if transmitted with suppressed carrier, 
can give extremely puzzling results if demodu¬ 
lated with an ordinary radio set. These possi¬ 
bilities should be taken into account if signals 
are found which do not seem to fit into the 
classes discussed below. 

As stated before, the spectrograph is of tre¬ 
mendous assistance in recognizing the nature 
of an unknown scrambling system. The ear can 
usually recognize the presence of time discon¬ 
tinuities. It can also usually recognize the pe¬ 
culiar quality which results from band-shifting 
systems. The exact nature of the scramble, 
however, is usually impossible to establish with 
the ear. Even scrutiny of the wave form may 
yield no clue. The strikingly graphic analysis 
provided by the spectrograph, however, usually 
takes the mystery out of the scrambling method 
immediately. 

Speech privacy systems having frequency 
subbands will show horizontal discontinuities 
or boundaries in their spectrograms. Similarly 
systems employing time division will show ver¬ 
tical boundaries. A considerable variety of sys¬ 
tems display both horizontal and vertical bound- 


'I 





DIAGNOSIS OF UNKNOWN SYSTEMS 


87 


aries. How to tell these different scrambling 
systems apart is the subject of the discussion 
and illustrations to follow. 

Measurements on Spectrograms 

Since an important part of the diagnosis 
procedure consists in determining the length 
of time elements and the location of frequency 
boundaries, let us first examine the procedures 
whereby the time and frequency scales of the 


The application of this method to 11-kc spec¬ 
trograms is not explicitly stated in the figure. 
A value of K for this condition can be found 
by the same formulas. This establishes the time 
scale for the 11-kc spectrograms. For the fre¬ 
quency scale the same pattern is used as for 
the 3.5-kc spectrograms. However, each hori¬ 
zontal striation is labeled with a frequency 
obtained by multiplying the normal frequencies 
by the ratio of the two K’s. 



Figure 44. Calibration of spectrograph scales. 


The upper spectrogram (narrow filter) shows all the odd har¬ 
monics of the 60-cycle input to a special harmonic generator. In the 
wide filter spectrogram (below) the striations represent a beat note 
of 120 cycles. 

At the left is a portion cut off and inverted. The fact that the 
harmonics can be lined up in this as well as other shifted positions 
illustrates the linearity of the frequency scale. At the right is a 
portion cut off and shifted downward by one component. Since the 
harmonics are odd, the base line will fall exactly between two har¬ 
monics if it represents exactly zero frequency. 

spectrograph can be established. The spectro¬ 
graph is equipped with a calibrating device 
which consists of means for producing a com¬ 
plex wave rich in harmonics from the 60-cycle 
power supply. Spectrograms of this wave made 
with both the 45-cycle filter and the 300-cycle 
filter are shown in Figure 44. If the power 
frequency is known, the horizontal and vertical 
striations in these patterns provide the time 
and frequency scales. If the power frequency is 
not known the scales may be established by the 
formulas given in the figure. This involves ad¬ 
ditional measurements with a stop watch. 


If the power frequency is exactly known, both the time scale and 
the frequency scale are determined by the two patterns above. If 
the power frequency is not known, the time scale factor can be 
determined by the equation K = LR/T inches per second, and the 
frequency hy F= KS/N. 

L = Total length of the spectrogram (circumference of the re¬ 
cording drum). 

R = Number of rotations of the drum in T seconds. 

S = Number of striations in N inches. 

Figure 45 shows how these scales can be 
used to measure the time and frequency bound¬ 
aries in a scramble. It will be noted that for 
measuring the time elements spectrograms 
made with the 300-cycle filter are best because 
they have sharper time boundaries. For meas¬ 
uring frequency boundaries the same filter must 
be used as was used in obtaining the scale. It 
may be noted here that in present models of 
the spectrograph, the wide filter has a different 
absolute location than the narrow filter and 
therefore should not be used to estimate the 
frequency of components or boundaries. 













































































































































88 


UNSCRAMBLING AND DECODING METHODS 


Illustrations of Scrambled Speech 
Spectrograms illustrating a large number of 
privacy system scrambles are shown in Figures 
46 through 65. In so far as possible, these 
spectrograms were obtained with actual work¬ 
ing models or systems. In some cases they were 


stroy the typical harmonic structure of speech 
leaving structureless patterns which cannot be 
interpreted. This indicates a distortion of the 
wave form. One of these systems, which had a 
repeating code and a synchronizing pulse, could 
be resolved by the method shown in Figure 55. 



Figure 45. Time and frequency measurements. 

Upper spectrogram, 45-cycle filter; lower spectrogram, 300-cycle which gives sharp time boundaries, comparing them with the stria- 

filter. The frequency boundaries are determined by comparing them tions obtained by making a spectrogram of the calibrating wave 

with the harmonics of the calibrating wave. These are all 120 cycles with the 300-cycle filter. Each one represents 1/120 second. Ten of 

apart, but the lowest is only 60 cycles from the base line. the above elements cover 70 striations. The length of each element 

The element length is best determined by using the 300-cycle filter is Vio x 'i'9i20 second. 


made with a laboratory setup simulating the 
systems under scrutiny. In a few cases also the 
illustrations were made by cutting up spectro¬ 
grams and rearranging the parts. It should be 
noted in these latter cases that the boundaries 
are unnaturally clear and sharp because in 
practice any discontinuity causes a transient 
which tends to obscure the true speech along 
the boundaries. 

It will be noted that some of the spectrograms 
in the illustrations were made with the 45-cycle 
filter and some with the 300-cycle filter depend¬ 
ing on what features were to be brought out 
most clearly. 

In some cases the spectrograms alone are 
not sufficient to determine the exact nature of 
the scramble. Certain systems completely de- 


No general rules, however, can be given for 
diagnosing this type of system. 

Systems not Illustrated 

Examination of Table 1 shows that there are 
a few scrambling systems which are not repre¬ 
sented in the illustrations. These will be dis¬ 
cussed in the following paragraphs. In most 
cases, the appearance of the spectrogram pat¬ 
tern which would result can be visualized by 
analogy with other systems. 

The phase-reversal system (A4) will produce 
a scramble indistinguishable from the multipli¬ 
cation system (HI) provided the phase re¬ 
versals occur at irregular intervals and about 
as rapidly as the crossovers in the coding wave 












DIAGNOSIS OF UNKNOWN SYSTEMS 


89 


involved in HI. It is probable that they would 
have to occur about that often to make speech 
unintelligible. 

The split-phase system (A5) involving car¬ 
riers 90 degrees apart was tried out in the lab¬ 
oratory. The output appears just as if two 
speech channels, or a speech channel and an 
interfering noise, were simply superposed and 
then modulated with a single carrier. 

The stepped displacement system (B2) is 
rather easy to visualize. There will be time 
boundaries, with two or more discrete condi¬ 
tions of displacement. Obviously, there are a 
great number of possible sequences, including 
the possibility of some of the conditions con¬ 
sisting of inverted displacement. 

The irregular wobbled displacement (B4) 
will be similar to B3 except that the wobble 
pattern will not be as simple. 

The continuously varied re-entrant displace¬ 
ment (C2) is practically impossible to simulate 
artificially, as was done with Cl. If Cl is thor¬ 
oughly understood, however, the appearance of 
a wobbled instead of stepped re-entrant condi¬ 
tion is not difficult to visualize. 

Nonrepeated-code TDS (F3) will have the 
same general appearance as repeated-code TDS. 
It may or may not have the synchronizing pulse. 
There will of course be no regularity in the 
patterns such as was pointed out in F2. 

The spectrogram for TDS plus inversion 
(Gl) is not difficult to visualize. Some or all of 
the elements might be inverted, as in A3. 

The systems listed in both G5 and G6 will 
show equally spaced time boundaries corre¬ 
sponding to the length of the elements. In G5, 
the harmonics would be spaced much farther 
apart than in normal speech, and show greater 
slopes and curvatures. Alternate elements 
would show rather consistent differences in 
frequency distribution and in the degree of 
slope or curvature. In G6, the harmonics would 
be spaced abnormally closely, and show very 
little slope or curvature. Words and spaces 
would be very long. There would be a horizontal 
boundary in the middle of the band, and the 
patterns in each half would appear like com¬ 


plete spectrograms, with vowel and consonant 
structures apparent. In both of these systems, 
if the elements were cut apart, they could be 
rearranged to form continuous speech with the 
time and frequency scales compressed or ex¬ 
panded from the normal condition. 

Level modulations (H2andH3) would hardly 
show up in spectrograms because of the level 
compression incorporated in the spectrograph. 
This has been verified experimentally. 

In J1 and J2, if the noise were sufficient to 
mask the speech effectively, the speech could 
not be seen in spectrograms. Patterns for J3 
and J4 are easy to visualize. If the noise spurts 
are sufficiently close together, however, they 
may produce a pattern like HI. As far as is 
known, J5 exists only on paper. 

In Vocoder types of scrambling systems the 
spectrograph would show only the channel sig¬ 
nals, which might be either amplitude or fre¬ 
quency modulated. For this type of scramble, 
oscillograms of the wave form of each separate 
channel signal provide the best means for diag¬ 
nosis and for decoding. A sample of such oscil¬ 
lograms, which was obtained from an actual 
Vocoder system, is shown in Figure 50. The 
various methods of scrambling such signals 
(Kl, K2, K3, K4) will produce discontinuities 
in these traces which are easy to visualize. A 
sample of K5 has not been available. 

Channel mixing (L3) can be done in various 
ways and at various speeds. It will not be easy 
to recognize if done rapidly. No actual systems 
are in use, as far as is known. 

It is felt that the above illustrations and dis¬ 
cussions cover the known scrambling methods 
fairly thoroughly. It is hoped that with their 
help any system which might be encountered in 
the future can be recognized. Certain additional 
spectrographic material appearing in Part I 
of the final report on Project 0-43^^ is useful 
to anyone wanting all possible data on the sub¬ 
ject. Part IP^ includes all the preliminary re¬ 
ports dealing with specific phases of the work 
carried out under the project. 

b Project C-43, Contract OEMsr-435, Western Elec¬ 
tric Company, Inc. 




90 


UNSCRAMBLING AND DECODING METHODS 



Figure 46. Illustrating modulation sidebands, 


The upper spectrogram shows speech modulated with a carrier of 
2,000 cycles. Note the symmetry of the pattern around this frequency. 
Each hai-monic, and each resonance area, is duplicated on both sides 
of the carrier. This clearly shows that the two sidebands are exactly 
alike, except that one is inverted. The recording from which these 
spectrograms were taken has somewhat attenuated the frequencies 
near the base line. 

The carrier itself is largely suppressed by the double balanced 
modulator: high level sounds, however, occasionally unbalance it 
sufficiently for the carrier to show through. 


In the lower spectrogram, the carrier frequency has been wobbled 
at a rather slow rate. Note that as the sidebands move up and down 
with the carrier, the harmonics remain parallel, as at point a, 
except when marked voice inflections occur, as at point b. 

In the clear space at the left, the wobbling carrier can be seen, 
together with its second harmonic. In the upper part of the spectro¬ 
gram at point a, the second harmonic of the carrier can also be 
seen, with its own set of sidebands. 

When the carrier frequency is low, as at points c and d, the lower 
sideband can be seen folding back; the folded back portion is right 
side up and overlaps the regular sidebands. 



Figure 47. Simple inversion. 


The upper spectrogram was made in the normal manner, showing 
inverted speech; the lower one was made with reversed oscillator 
sweep, producing a mechanically inverted spectrogram in which the 
speech appears right side up. 

This sample contains harmonics with marked curvatures. These 
are voice inflections, and their occurrence can easily be recognized 
by ear. In general, samples with such voice inflections should be 
captured because they are most useful for diagnosing scrambling 
systems. 

In the upper spectrogram, at points a and d, note how the curva¬ 
ture of the harmonics is least at the highest frequencies and pro¬ 
gressively greater toward the lowest frequencies. Similarly, at points 


b and c, the slopes of the harmonics are least at the top and greatest 
at the bottom of the spectrogram. This is directly the reverse of 
normal speech and definitely indicates inversion. The lower spectro¬ 
gram illustrates the normal slopes and curvatures. 

There is obviously a low-pass filter in the system, at about 3,000 
cycles, as indicated by the rather abrupt change in intensity. Such a 
filter is normally used to cut off the upper sideband. It is usually 
also designed to cut off the carrier. In this case, its cutoff frequency 
is lower than the carrier frequency. This shows up at points a and b 
in the harmonics, fading out before they completely flatten out. 
However, the inversion frequency is not far from 3,000 cycles, be¬ 
cause the slopes ai*e substantially zero as they approach this fre¬ 
quency. 



































DIAGNOSIS OF UNKNOWN SYSTEMS 


91 



Figure 48. Alternate inversion. 


Here the speech is divided up into sections by sharp vertical 
boundaries. When the individual elements are examined, a, b, and c 
show unmistakable signs of inversion. Elements d, e, and / are 
definitely not inverted. 

The regularity of the dark areas in the lower spectrogram suggests 
that only two conditions are involved. Note also that in general, 
where slopes can be clearly discerned, the harmonics slope in oppo¬ 


site directions in adjacent elements. These indications point to 
alternate straight and inverted transmission. 

This diagnosis could be confirmed by making a mechanically 
inverted spectrogram and matching together alternate pieces from 
the two spectrograms. 

The switching intervals are irregular, with no repetition apparent 
within the time covered by this spectrogram. Additional spectro¬ 
grams, covering a longer period, might show a repeated cycle. 



Figure 49. Fixed displacement. 


In the upper spectrogram, the speech band has been displaced 
from its normal location by 1,000 cycles; in the lower one, by 2,000 
cycles. Recognition of this condition is aided by familiarity with the 
appearance of normal speech in spectrograms. All vowels have 
characteristic resonant areas close to what would be the base line 
(zero frequency) in normal speech, and the glides such as occur at 
a and b tend to start from this region. 

At c, the harmonics look as though they could meet at a point if 
extended to the right. This point would be the true base line. 


A displacement of this type could be produced by modulating with 
a carrier of 1,000 or 2,000 cycles, and suppressing the lower (in¬ 
verted) sideband with a high-pass filter. In practice, however, a 
double modulation process is used, because the displacement may be 
changed at will without changing the filter cutoff. 

In both spectrograms, a small amount of lower sideband can be 
seen. This incompletely suppressed sideband would look the same 
whether produced by single or double modulation. 






























92 


UNSCRAMBLING AND DECODING METHODS 



Figure 50. Wobbled displacement. 


The upper spectrogram shows an example of wobbled inversion. 
Note the occurrence of harmonics symmetrically disposed about a 
suppressed carrier frequency which is varying between 2,500 and 
3,500 cycles. Note how the harmonics remain essentially parallel. 
At points a and b the sidebands appear to consist only of low-fre¬ 
quency noise. The carrier wobble is irregular in shape but regularly 
repeated in time. 

If there is a low-pass filter in the system, its cutoff frequency is 


higher than 3,500 cycles. Note that if it were lower, it would occa¬ 
sionally cut off some of the wanted (lower) sideband. 

The lower spectrogram shows a wobble covering a much wider 
frequency range. The lower sideband dips below the 3,500-cycle 
range of the spectrogram only part of the time. This would cer¬ 
tainly be diagnosed as a band displacement system involving double 
modulation. If it were encountered in practice, wide-band spectro¬ 
grams would be used to determine the exact displacement. 



Figure 51. Re-entrant inversion. 


The upper spectrogram was artificially produced by cutting up 
and rearranging a spectrogram of simple inversion. 

There are vertical boundaries about 275 msec apart. There are 
also horizontal boundaries, but these are not continuous. 

Elements 3 and 6 show no horizontal boundaries, but they show 
the signs of simple inversion. All the other elements show horizontal 
boundaries, with higher slopes above the boundary than below. If 
the two portions of each element were interchanged, the slopes would 
be in the correct order (for inverted speech). 

It can also be seen that if the elements were thus rearranged, the 


harmonics of each element would match those of the preceding and 
following elements. This, of course, should be confirmed in practice 
by trial. 

These configurations would be produced by re-entrant inversion. 
In terms of these spectrograms, this process results in inverting 
successive elements about frequencies of 1,000, 2,000, and 3,000 cycles, 
respectively, removing the upper sideband, and replacing it with 
that portion of the lower sideband extending below about 200 cycles. 

The lower spectrogram is a duplicate of the one above. The 
boundaries have been marked off to show that the elements rear¬ 
ranged, as suggested above, would form continuous inverted speech. 



























































DIAGNOSIS OF UNKNOWN SYSTEMS 


93 



Figure 52. Fixed split-band scramble. 


These are two samples with the same code. Note the horizontal 
discontinuities in the frequency distribution of energy, which can 
best be seen by looking endwise along the patterns. These are the 
filter boundaries. This system shows five bands, covering the range 
from 250 to 3,000 cycles. In split-band systems, the subbands are 
generally equally wide, for practical reasons. 

There are no vertical discontinuities, other than the normal 
sequence of words and spaces. 

Note that the harmonics slope or curve in opposite directions 
within a word or syllable. This indicates that some of the subbands 
have been inverted. The first and fourth bands are clearly normal, 
the others are inverted. This shows up clearly at point a. 


Note that the fourth band shows the least slope or curvature: 
this must have been the lowest band originally. The middle band 
shows the most slope or curvature; this must have been the highest 
originally. The others can be similarly located, combining the indi¬ 
cations from all the indicated points. Any one point is sometimes 
misleading due to the proximity of a harmonic to the filter bound¬ 
ary, as in the top band at a. 

At points d and e there is a double inflection. This can be mis¬ 
leading, unless the slopes are estimated for simultaneous instants. 
The vertical line was drawn as a guide. 

The code is 4, 2', 1, 5', 3', the primes denoting inversion. 



Figure 53. Rapidly switched split-band scramble, example 1. 


These spectrograms show both horizontal and vertical boundaries. 
However, the level of the elements as a whole shows a rather smooth 
flow, as of words and spaces. Note the clear space ahead of the first 
word. These indications suggest that the elements have not been 
shuffled in time. 

The presence of band shifts, however, is quite obvious. Harmonics 


can be seen sloping in both directions within the elements, particu¬ 
larly in the first word group. 

The checkerboard effect in the lower spectrogram suggests that 
only two codes were used alternately. This is corroborated by the 
fact that the middle band shows no vertical discontinuities, indicating 
that it was never switched. 





















94 


UNSCRAMBLING AND DECODING METHODS 



Figure 54. Rapidly switched split-band scramble, example 2. 


Here there are both horizontal and vertical boundaries. There is 
no evidence, however, that the elements have been time shifted. 
There are unbroken clear spaces as at a, the long word groups such 
as h show no abrupt changes in level or in pitch, and elements 
having different characteristic appearance, such as those at c and d, 
are not intermingled. 

There is abundant evidence of band shifts: harmonics sloping in 
opposite directions, indications of inversion, and abrupt changes in 


the resonance areas. Close scrutiny of long word groups such as h 
shows that several codes are being used, although it would take 
several samples to establish just how many. 

This sample illustrates that if the codes are rapidly shifted, any 
one element seldom contains enough clear information to determine 
which code applies to it. However, if accumulated information about 
the system can be brought to bear, two clear bands may sometimes 
be sufficient to identify the code. 






Figure 55. Time division multiplex. 


At first glance these look like split-band scrambles. The horizontal 
boundaries are quite evident. The bands are 600 cycles wide, begin¬ 
ning at 400 cycles. It will be noted, however, that all four bands 
look alike, except that alternate ones are inverted. Otherwise the 
slopes and curvatures are alike in all bands. An outstanding example 
is at point a. There is no gradation in slope. 

The components are not uniformly spaced within a band, and 
they frequently go in both directions within a band, as at b and c. 


These are the chai'acteristics of TDM scrambling. In the particular 
system illustrated, the frequency range was divided into four bands, 
and all were modulated down to the lowest frequency. The switching 
rate was 600 per second, but the entire band was then modulated 
up 400 cycles, to avoid having the lowest sideband extend to fre¬ 
quencies too low for transmission channels to handle. 

It is characteristic of TDM to produce upper and lower sidebands 
around the switching frequency and around its odd multiples. The 
sidebands differ only in the phases of their components. 


















DIAGNOSIS OF UNKNOWN SYSTEMS 


95 



Figure 56. Time division multiplex with noise channel. 


This is the same TDM system as in Figure 55, but a band of noise 
has been added to increase the privacy. One half of the highest of 
the four frequency bands into which the speech channel was divided 
has been filled with thermal noise. In the upper spectrogram of 
the figure, this thermal noise was steady; in the lower spectrogram. 


the noise was turned on and off about four times per second. 

Note that although the noise was introduced into only one sub¬ 
band, it appears in each of the four sidebands in the above patterns. 
This shows that in TDM, each sideband contains components from 
each subband. 



Figure 57. Subbands 

Here there are horizontal boundaries, but the filters apparently 
do not cut off very sharply because the bands appear to overlap 
occasionally. 

Note the staircase effect in the upper spectrogram, each syllable 
in the uppermost band appearing somewhat later in the middle 
band, and still later in the lowest band. This condition has been 
rectified in the lower spectrogram by cutting the frequency bands 


variously delayed. 

apart and shifting them relative to each other, thereby restoring the 
normal appearance of words and spaces. 

There has been no shifting or inversion of the subbands. Note that 
the filter crossovers have been made very deep, as evidenced by the 
gaps between bands, and the lowest band has been severely cur¬ 
tailed in width, probably in an effort to reduce the amount of 
intelligence which may be gained by listening to any one band. 






























96 


UNSCRAMBLING AND DECODING METHODS 



Figure 58. Combination of TDS and 

In this sample the evidences of band shifting are clear enough. 
That several different codes are used is suggested by the irregular 
distribution of the resonant areas over the frequency range. It is 
more conclusive, however, to examine the slopes of the harmonics 
in the upper spectrogram. At a, for instance, the lowest band shows 
more slope than either the second or fourth; at c, the opposite is 
true. At o, the harmonics in the second and third band slope in the 
same direction; at b, in opposite directions. To tell how many codes 
are used wmuld require additional samples. 


rapidly switched split-band scramble. 

Evidences of time shifting are also clear. Elements a and b, for 
instance, are both strong, but are surrounded by gaps. At d, an 
element with energy distributed over the whole frequency range is 
surrounded by elements with entirely different distribution. There is 
also a marked difference in pitch between d and the surrounding 
elements. An even more marked change occurs at e. 

This scramble, therefore, is the result of both band shifting and 
time shifting. It differs from a complete two-dimensional scramble 
in only one respect, which is described in a separate illustration. 



Figure 59. Nonsynchronous combinations of TDS and split-band scramble. 


These are two combinations of TDS and rapidly switched split- 
band scrambles. They differ from the previous illustration in that 
the two switching systems are independent. The split-band code is 
changed at intervals of about 40 msec, whereas the length of the 
TDS elements is about 34 msec. 

Each of these switching systems produces its own set of vertical 
boundaries. The distance between successive boundaries in the 
scramble, therefore, varies irregularly from a value corresponding 
to 34 msec down to zero. 


In the upper spectrogram, the speech was first subjected to TDS, 
and then to the split-band scramble; in the lower spectrogram, the 
two scrambles were applied in the reverse order. 

Only two split-band codes were used, alternately. Since the second 
scramble tends to hide the first, the upper spectrogram shows the 
characteristic checkerboard effect noted in a previous illustration. 
In the lower spectrogram, the checkerboard effect is broken up by 
the TDS. 


















DIAGNOSIS OF UNKNOWN SYSTEMS 


97 



Figure 60. Test for two-dimensional scramble. 


Spectrograms which serve to illustrate how a two-dimensional 
scramble might be recognized. The scramble contains both time 
shifts (TDS) and band shifts (rapidly switched split band). Note, 
however, that in either of these scrambling systems, and in the 
combination of both, elements which are simultaneous in the scram¬ 
ble (that is, subbands within any vertical section) were also 
simultaneous in the original speech. 

In this example, therefore, a decided tendency may be seen for 
the character of a vertical section to remain constant over the 
frequency range. That is, low level elements are low all over the 
frequency range; high level elements tend to be high all over. 

Furthermore, subbands from voiced sounds do not occur in the 


same vertical sections with subbands from unvoiced sounds. Voiced 
sound may be recognized by the presence of harmonics in the 
45-cycle spectrograms, and regular vertical striations in the 300- 
cycle spectrograms. 

The most conclusive test for a two-dimensional scramble, however, 
is based on the fact that there will be differences in pitch within 
a vertical section. This can easily be tested as illustrated above. 
The spectrogram is cut down the middle of a vertical section, and 
the pieces shifted by one harmonic in either direction. If there is 
no change in pitch, the harmonics will still match all over, as above. 
If there is a change in pitch, the shift which is correct for one 
subband will be wrong for another. Two-dimensional scrambles, 
therefore, will not pass the above test. 



Figure 61. Speed wobble 


The curvatures of the harmonics in the upper spectrogram look 
like voice inflections, except that they are abnormally frequent and 
rapid. The resonance areas, as shown best in the lower, also show 
a marked degree of curvature. Also, marked correlation may be 
noted between the resonance areas and the pitch, that is, they reach 


their high and low points simultaneously. In normal speech, the 
frequency and trend of the resonance areas are independent of the 
pitch trend. Wobbling the speed of a phonograph record or magnetic 
tape, however, multiplies the frequency of resonances as well as mul¬ 
tiplying the pitch. These spectrograms wei’e produced in this manner. 






































98 


UNSCRAMBLING AND DECODING METHODS 



Figure 62. 

The above spectrograms were produced artificially by cutting up 
and rearranging spectrograms of normal speech. 

The upper spectrogram shows the speech transmitted in sections 
about 160 msec long, each section transmitted backwards. If the 
sections are as long as this, the condition can be recognized by 
familiarity with the normal speech formations, that is, by the way 
words normally start and end, and by the trend of the resonant 
areas. The slopes and curvatures of the harmonics, however, look 
perfectly normal. 

If the elements are cut apart and matched, it will be found that 


Backwards. 

the right-hand edge of each element matches the left-hand edge of 
the preceding element. This can be seen by inspection of the above 
example. The order of the pieces will be completely reversed after 
matching. If all the pieces are inverted, however, they will be found 
to match in their present order. 

The lower spectrogram shows the same material, but in this case 
alternate elements are transmitted forwards and backwards. It will 
be found that none of the elements can be matched together at all. 
To match, alternate elements must be taken from a mechanically 
inverted spectrogram, as described in another illustration. 



Figure 63. Multiplication. 


The upper spectrogram shows a continuous noise, with several 
words or syllables showing through. Counting the harmonics shows 
that the fundamental of the noise is about 100 cycles. 

Examination of the signal with an oscilloscope shows that the 
noise consists of short pulses about 10 msec apart. These can be 
removed by a blanking circuit. 

The lower spectrogram shows a sample (not the same as the one 
above) without the pulses. The outstanding characteristic, as in the 
sample above, is an almost complete lack of the harmonic structure 
of normal speech. Also, the energy is distributed more or less evenly 
over the frequency range for each word or syllable. There are no 
characteristic resonance areas. 

There are no regular boundaries, either vertical or horizontal. 


The sequence of words and spaces looks normal in the spectrogram, 
and has the normal cadence of speech to the ear. 

These characteristics are to be expected when the scrambling 
system operates on the wave form directly. In this particular system, 
the speech wave was multiplied by a coding wave. The latter was 
repeated 100 times per second, with a pulse between each cycle. It 
is obvious that a high degree of synchronism is required to remove 
the coding wave at the receiving end, which accounts for the high 
frequency of the synchronizing pulses. 

It may be noted that phase reversal (at a sufficiently high and 
irregular rate to achieve privacy) is also essentially a multiplication 
process, except that the coding wave has no values other than plus 
and minus unity. Spectrograms of such a system would be expected 
to look like the above. 





























































DIAGNOSIS OF UNKNOWN SYSTEMS 


99 



Figure 64. Time division channel mixing. 


Here two talking circuits have been switched between two trans¬ 
mission channels on a time division basis, at 300-msec intervals. The 
vertical discontinuities can usually be seen, but point a is an out¬ 
standing example of apparent continuity in pitch, inflection, and 
resonance. 

The ear can usually recognize the fact that two voices are present. 


at least at this switching rate. If the voices are nearly alike, or if 
recorded samples of the same voice are used, the nature of the 
scramble can be determined by cutting the pieces apart and attempt¬ 
ing to rearrange them into continuous speech. This, of course will 
be found impossible in channel mixing. Another transmission channel 
should be found with the complementary elements. 









Figure 65. Subband channel mixing. 


Here the harmonics occasionally curve in different directions, as 
at point a. A horizontal discontinuity is quite apparent at the 
indicated frequency, above which there are changes in pitch. These 
are not always readily apparent to the eye, but can be established 
by measurements. 

In general the syllables seem to begin and end at different times 
in the two bands. Formations such as c and d do not occur in 
normal speech. 

No vertical discontinuities are apparent in either band, which 
indicates that if any time delays are involved, they apply to the 


whole bands. Yet, by trial, the speech in the upper and lower bands 
cannot be matched by shifting the bands relative to each other. 

It is apparent that two talking circuits are being switched be¬ 
tween two transmission channels. Another channel should be found 
which will contain the complementary subbands. This was produced 
by a laboratory setup. In practice, to obtain sufficient privacy, it 
would probably be necessary to combine this subband channel mixing 
with time division channel mixing illustrated elsewhere. 

Point b, as a matter of interest, marks an outstanding example 
of apparent continuity in both pitch and slope. 











































Chapter 5 


DECODING PROJECTS 


SPEECH PRIVACY PROBLEMS 

S tudies and analyses of several privacy 
systems of special interest to the Bureau 
of Ships were made under Project 
a continuation of the work carried forward 
under Project C-43. In each problem the prin¬ 
cipal objectives were the determination of the 
security afforded by and the transmission per¬ 
formance of the privacy system in question. 


Nature and Scope of the Work 

The material submitted for study and analy¬ 
sis under this project comprised working models 
of two privacy systems, recordings of speech 
scrambled by three privacy systems, and paper 
proposals for two systems. 

Security evaluations were made under favor¬ 
able laboratory conditions. It was assumed that 
the enemy (1) was thoroughly familiar with 
the speech privacy system, (2) had the neces¬ 
sary intercept, recording, and decoding equip¬ 
ment, (3) had trained personnel, (4) was in a 
position to receive adequate signals, and (5) 
was completely organized so that no time would 
be lost in obtaining and making use of intelli¬ 
gence from the decoded message. The security 
ratings assigned to the several systems evalu¬ 
ated did not take into consideration any prac¬ 
tical difficulties which might be encountered in 
the field or under combat conditions where the 
work of intercepting, diagnosing, decoding, and 
obtaining intelligence from scrambled messages 
must be carried on under stress. 

The following assignments of work on this 
project were authorized by Division 13: 

1. British Modulator Type 2C (manually 
switched) working models. 

2. British Two-Dimensional Privacy System, 
recording. 

3. British Modulator Type 2C (rapidly 
switched), recording. 

»Project 13-106, Contract OEMsr-1440, Western 
Electric Company, Inc. 


4. New Zealand Switched-Band Privacy Sys¬ 
tem, recording. 

5. New Zealand Switched-Band Privacy Sys¬ 
tem, working models. 

6. Proposals of L. E. Gabrilovitch for pri¬ 
vacy systems. 

Summary of Results 

The results of the work done on the several 
assigned problems are given in detail in Reports 
1 to 5, inclusive, which form an appendix to 
the final report of the project.^^ These reports 
cover the six assignments listed above with the 
fourth and fifth assignments, relating to the 
New Zealand switched-band system, being com¬ 
bined in one report. All of the systems consid¬ 
ered under this project were of the short-term 
privacy variety. Of these, the British two- 
dimensional system appeared to be the most 
promising. 

Brief Resume of Systems 

British Modulator Type 2C (Manually 
Switched), Working Models, This system pro¬ 
vides four fixed speech scrambling conditions, 
each involving either one or two simple modu¬ 
lating processes; the choice of any one of the 
four scrambling conditions or clear speech is 
under the control of a manual switch. 

When a receiving unit, or its equivalent, is 
at hand, there is no difficulty whatever in dis¬ 
covering the proper decoding condition in a 
matter of seconds. The security afforded by the 
system is, therefore, almost nil. 

The fixed-code scrambles can be demodulated 
satisfactorily by a single modulation process 
without filters, it being necessary only to use 
the appropriate frequency of the demodulating 
carrier. For this reason it is possible to obtain 
intelligence from radio transmissions of the 
scrambled speech by means of an ordinary type 
of radio receiver equipped with a beat-fre¬ 
quency oscillator. The efficacy of this method 
will, of course, depend upon having adequate 
relative stability of the radio carrier and the 
beat-frequency oscillator. 


100 



SPEECH PRIVACY PROBLEMS 


101 


These units were well constructed and oper¬ 
ated satisfactorily from the standpoint of over¬ 
all speech quality. 

British Two-Dimensional Privacy System, 
Recording. This system utilizes both frequency- 
and time-division scrambling. It employs three 
frequency bands and ten time elements of 0.065- 
sec duration in a repeated code. The time delay 
in one direction of transmission (exclusive of 
delay of the transmission path) is 0.65 sec. 

The evaluation of this system was based on 
the study of one recording bearing a single 
sample of scrambled speech, together with sam¬ 
ples of clear speech and speech which had been 
coded and decoded for comparison. An evalu¬ 
ation based on such limited data is necessarily 
tentative and should be supplemented by tests 
on working models. 

The speech scrambled by this system appears 
to be invulnerable to direct listening and to 
other noncryptographic attacks. It is, however, 
vulnerable to cryptographic attacks and a 
working solution of the code can probably be 
obtained in a matter of 3 or 4 hours. With 
a model of the receiving equipment at hand, it 
is conceivable, although it was impossible actu¬ 
ally to try it, that a substantial amount of 
intelligence could be obtained in the order of 
half an hour. This latter method would involve 
the use of spectrograms from which sugges¬ 
tions are obtained for setting up partial decodes 
on the receiving unit. 

The most noteworthy weakness in the British 
two-dimensional system appears to be the use 
of a fixed repeated code. The addition of code 
changing means would increase the crypto¬ 
graphic security very greatly. 

The quality of the restored (or decoded) 
speech presented on the recording compared 
favorably with the clear (or uncoded) speech 
on the same recording. 

British Modulator Type 2C (Rapidly 
Switched) Recording. This system is the same 
as the Type 2C discussed above except that 
means are provided for rapidly switching from 
one scrambling condition to another and that 
clear speech is used as a fifth scrambling condi¬ 
tion. The order in which the scrambling con¬ 
ditions are selected is predetermined according 
to a code which repeats after a sequence of 20 


such selections, each enduring for approxi¬ 
mately 0.065 sec. 

A code-switching mechanism for use with 
this system was promised, but was never re¬ 
ceived. This would have made it possible to 
make a more positive evaluation of the system 
than can be made from the recording of scram¬ 
bled speech. In fact, the results obtained from 
noncryptographic attacks on the recorded 
scramble are considerably at variance not only 
with what could logically be expected, but also 
with the results obtained with working models 
of a very similar system.^*^ 

Repeated listenings directly and also through 
a two-path superposition circuit to the recorded 
scramble yielded several words and phrases but 
very little intelligence. Similar tests on the New 
Zealand system (working models) yielded, on 
the average, 40 per cent intelligence to direct 
listening and 80 per cent intelligence with the 
two-path circuit. 

Repeated listenings through an automatic 
analyzer-decoder circuit yielded approximately 
60 per cent of the intelligence from the recorded 
scramble of the British Modulator Type 2C 
rapidly switched system. This same procedure 
yielded practically 100 per cent intelligence on 
the New Zealand system. 

A cryptographic solution of the repeated- 
code sequence used in making the recorded 
scramble can be determined by inspection of 
two spectrograms in about 15 min. 

The quality of restored speech on the record¬ 
ing compared favorably with the clear speech 
on the same recording. 

New Zealand Switched-Band Privacy System, 
Recording and Working Models. Fundamen¬ 
tally, this system is very similar to the British 
Modulator Type 2C, rapidly switched, and dif¬ 
fers in what appears to be only minor details: 
The inversion frequencies in the scrambling 
circuits are somewhat different; the duration 
of each of the rapidly switched scrambling 
conditions is 0.043 sec (rather than 0.065 sec) ; 
a sequence of 18 selections of scrambling con¬ 
ditions (rather than 20) comprises the coding 
cycle. The New Zealand system is equipped 
with an applique unit for automatically chang¬ 
ing the code each cycle for a total of 625 cycles, 
or for a period of about 8 min, before repeating. 



102 


DECODING PROJECTS 


A recording of two samples of speech scram¬ 
bled by this system (using repeated code) was 
received for analysis and most of the intelli¬ 
gence was obtained by noncryptographic meth¬ 
ods. Somewhat later, the scrambling equipment 
for two terminal units was received and was 
set up for tests and demonstration as a two- 
way privacy system. 

The security afforded by this system is very 
low for military purposes and is inconsistent 
with its size and weight. 

Repeated listenings directly to the scramble 
yielded on the average about 40 per cent of the 
intelligence to experienced observers; repeated 
listenings to the scramble through a two-path 
superposition circuit yielded about 80 per cent 
of the intelligence. A repeated code can be 
determined by an aural method, using the ter¬ 
minal equipment, or its equivalent, in about 
7 min. An automatic analyzer-decoder circuit 
yielded at least 50 per cent of the intelligence 
from either a repeated or a nonrepeated 
(8-min) code sequence on the first listening 
and practically all the intelligence with few 
additional listenings. 

By cryptographic methods, a repeated code 
can be determined in about 20 min and a non¬ 
repeated code (including the starting point of 
the automatic code-changing unit) can be de¬ 
termined in about 1 hr. 

Mechanically and electrically the units oper¬ 
ated satisfactorily; the intelligibility of the 
restored speech was good but the quality, 
though fairly good, was somewhat inferior to 
what might be achieved with improvements in 
design. 

Proposals of L. E. Gahrilovitch for Privacy 
Systems. Of two proposals by Gabrilovitch, the 
first, described as a “Screen Secrecy Set with 
Narrow Audio Band,” appeared to require 
considerable equipment to obtain only a very 
limited degree of security with probably poor 
transmission performance and a sacrifice of 
operating range. 

The second proposal, described as a “Phase 
Varied Inverter-Distorter” (simplified secrecy 
set), although similar in basic principle to the 
RCA-Bedford system developed under Project 
0-54^^ offered, theoretically, some possibilities 
of obtaining a fairly compact and lightweight 


set having somewhat better restored speech 
quality than the Bedford system. There were, 
however, a number of questions regarding the 
degree of security, coding possibilities, and 
practicability of some of the electronic proc¬ 
esses. 

When, after study, it appeared fairly evident 
that the development of the second proposal 
would tend more and more to duplicate that of 
the Bedford system and would offer few, if any, 
advantages over the latter when completed, it 
was recommended that further study of this 
proposal be discontinued. 

General Conclusions and Remarks 

In the course of the work done under this 
project, a number of conclusions were reached 
regarding the systems under consideration and 
their evaluation based on the use of working 
models as contrasted with phonograph record¬ 
ings. 

Switched-Band Systems. Of the systems con¬ 
sidered, vulnerability to repeated listenings 
directly to the scramble is attributed to the 
inherent lack of privacy in some of the five 
speech-scrambling conditions. The average in¬ 
telligence obtained in listening to the five fixed 
scrambles is 40 per cent, which is approxi¬ 
mately the same as obtained (on the average) 
when the scrambling conditions were rapidly 
switched. The need for scrambling conditions, 
each having an adequate degree of privacy, is 
obviously indicated. 

The high yield of intelligence obtained from 
superposition listening is attributed mainly to 
the fact that some of the five scrambling con¬ 
ditions are not mutually private and effectively 
decode one another (Codes A and B in the New 
Zealand system and Codes 1 and 3 in the British 
system). This effectively reduces the available 
number of scrambling conditions. Hence, the 
five scrambling conditions should be not only 
inherently private but also mutually private. 

The vulnerability of the systems to either 
direct or superposition listening is independent 
of whether a repeated or nonrepeated code is 
employed. 

The use of a repeated code makes the system 
very vulnerable to methods of cracking wherein 
the code is to be determined. It is necessary to 



SPEECH PRIVACY PROBLEMS 


103 


decode only one cycle when successive cycles 
can be used to obtain confirmation. 

When the system is not vulnerable to non¬ 
cryptographic attack, the use of nonrepeated 
coding increases privacy. If the coding is truly 
random, it is necessary to decode each indi¬ 
vidual cycle with no opportunity for confirma¬ 
tion from successive cycles. 

Two-Dimensional System. This type of sys¬ 
tem involving both frequency and time division 
scrambling affords more security than can be 
obtained by using either method of scrambling 
alone. In the case of the British system con¬ 
sidered, the privacy would have been materially 
increased by using a nonrepeating code, more 
frequency bands, and shorter and more time 
elements. 

Masking Systems. Systems of this type em¬ 
ploy a screen of noise overlaying the signal to 
be masked. This is accomplished in one system 
by modulating the masking and masked signals 
on a split-phase subcarrier. The discrimination 
between these two signals at the receiver re¬ 
quires absolute synchronization and proper 
phasing of the demodulating carrier. Since this 
is difficult to achieve in practice, because of 
the distortions appearing in the transmission 
channel, the restored speech will be of poor 
quality, being distorted and noisy. 

The relatively large amount of power re¬ 
quired for the masking signal reduces the effi¬ 
ciency of the radio transmitter in that smaller 
transmitting ranges are obtained for a given 
amount of output power. 

Bedford Type Systems. Systems of this type, 
of which the phase-varied inverter-distorter 
system proposed by Gabrilovitch is one, depend 
upon the modulation of speech by a complex 
coding wave to obtain privacy. Clear speech is 
obtained at the receiver by demodulation of the 
scramble with an accurately synchronized de¬ 
coding wave which effectively is the reciprocal 
of the coding wave. 

To avoid the possibility of partially cracking 
the scramble by demodulating it with a single 
frequency, it appears to be necessary that the 
complex coding wave have no predominant 
frequency components but, instead, should have 
a fairly uniform spectrum of at least several 
hundred cycles width within the limits of the 


speech band. The resulting band width of 
the scrambled speech exceeds the width of the 
speech band by an amount equal to the highest 
frequency in the coding wave. It follows, then, 
that either the band width of the channel con¬ 
veying the scrambled speech must be wider 
than for normal speech bands, or the original 
speech band must be made narrower than nor¬ 
mal if distortion is to be avoided. 

Since an accurately synchronized decoding 
wave of proper phase is required for deriving 
clear speech at the receiver, this system is sen¬ 
sitive to distortions in the transmission channel. 
The restored speech should not, however, be as 
noisy as that of the masking systems. In the 
Bedford type systems, imperfect demodulation 
yields unwanted products which are propor¬ 
tional to the speech energy rather than to the 
relatively large masking energy. 

Synchronization by means of a continuous 
modulated wave is believed to be superior to 
synchronization by pulses as proposed in the 
RCA-Bedford system. In the latter instance, 
the wave form of the transmitted pulse is both 
important to the proper operation of the system 
and sensitive to distortions over a large part of 
the band of the transmitting channel. 

Evaluation of Security of Systems from Re¬ 
cordings. Phonograph recordings of speech 
scrambled by a privacy system provide a less 
desirable means for evaluating the security of 
a privacy system than do working models of 
the system. The results of analyses based on 
phonograph recordings can be used for deter¬ 
mining the nature of the privacy system and 
the code but even though the quality of the 
recording is good, difficulty may be experienced 
in direct or superposition listening tests. 

Very often it is found that recordings, which 
are considered moderately good for clear speech 
are surprisingly inadequate for storing scram¬ 
bled speech for subsequent analysis and res¬ 
toration. This appears to be due to (1) har¬ 
monic distortion, which, when not too great, 
passes unnoticed in clear speech, and also to 
(2) irregular speed variations (in either the 
recording or reproducing systems) which pre¬ 
vent precise synchronization necessary in some 
privacy systems. However, the fact that a high 
quality recording is required in cracking a 



104 


DECODING PROJECTS 


given privacy system, is in itself, of consider¬ 
able practical importance in evaluating the 
system. 

The most effective cracking techniques often 
involve the use of a receiving unit, or its 
equivalent. When only recordings are available 
for analysis, it becomes necessary either to 
build an equivalent receiver or merely to specu¬ 
late on what might be done with a working 
model. Neither of these alternatives is very 
satisfactory. It is, therefore, highly desirable 
whenever possible, that evaluations be made by 
tests on working models. 

5 2 FIELD DECODING EQUIPMENT 

The experience gained with the sound spec¬ 
trograph in Projects C-32 and C-43 resulted 


in the recommendation that the device be re¬ 


designed for field use and included in field 
decoding kits at radio intercept stations. Under 
Project 13.3-86*^ eight of the units were built, 
three for the Signal Corps, three for the Navy, 
and two for the British under Lend-Lease 


requisition. Together with operating and main¬ 
tenance instructions, these eight spectrographs 
(D-165-529) were delivered between January 
and May 1, 1944. 


The units had the following weights and 


dimensions 


Unit Weight 

Recorder 68 lb 

Amplifier-analyzer 64 lb 

Rectifier 56 lb 


Dimensions 

(Inches) 

171/2X161/2X11% 

20%xll%xl4% 

20%xll%xl0% 


b Project 13.3-86, Contract OEMsr-1110, Western 
Electric Company, Inc. 




Chapter 6 

FACSIMILE PRIVACY SYSTEMS 


INTRODUCTION 

T he scrambling systems heretofore de¬ 
scribed have related to the transmission of 
speech. Another important form of communi¬ 
cation, however, is graphic copy such as maps, 
drawings, and photographs, where a facsimile 
of the original subject copy is to be transmitted 
to a remote point by wire or by radio. In war¬ 
time, of course, it is as important that privacy 
be attained in this form of communicated in¬ 
telligence as in the transmission of the human 
voice. 

Two projects under Division 13 were con¬ 
cerned with the general problem of scrambling 
graphic copy. Project C-7S^ being a survey of 
all existing and proposed systems and Project 
13.3-97'^ describing a system for scrambling 
the copy before scanning it for transmission by 
radio or wire. 


6 2 FACSIMILE PRIVACY 

^ ^ ^ Statement of the Problem 

At the time of this project, all concerned 
agreed that facsimile had attractive possibil¬ 
ities as communication means, but up to that 
time it had been handicapped by lack of privacy 
for military use. Considerable information on 
the several phases of the subject existed in 
various places but no attempt had been made 
to consolidate it. Accordingly Division 13 au¬ 
thorized Project C-73 which had as its object 
the survey of the general field of facsimile pri¬ 
vacy and the preparation of a report on the 
subject. 

The project was completed between December 
1942 and July 1943. A final report^^ was pre¬ 
pared on June 7, 1943, and a much abridged 
report of this final report on October 15, 1943. 
The summary to follow is taken from the 

a Project C-73, Contract OEMsr-837, Radio Corpora¬ 
tion of America. 

^ Project 13.3-97, Contract OEMsr-1202, Faximile, 
Inc. 


abridged report. Since it is the basis of any 
further work in this subject, this abridged 
report is summarized in some detail. The com¬ 
plete final report gives essential details of non¬ 
private systems and furnishes complete back¬ 
ground. 

The directive covering Project C-73 called 
for: 

1. A brief summary of basic facsimile mech¬ 
anisms and modes of transmission. 

2. Investigation of the degree of privacy ob¬ 
tainable. 

3. Adaptability of telephonic privacy systems 
then existing and those under development. 

4. Evaluation of means for cryptanalysis of 
various graphic privacy systems the enemy 
may use. 

5. Suggested design of the most useful type 
of equipment for both privacy transmission 
and cryptanalysis. 

The contractor made contact with all known 
individuals and organizations in the United 
States who might be able to contribute sugges¬ 
tions which would be useful in providing fac¬ 
simile privacy. A complete report on these 
contacts is given in the appendices of the final 
report. At the time of the survey, telephonic 
scrambling methods were under active study 
by Division 13 contractors and by the Military 
Services, but up to that time no attempt had 
been made to use telephonic systems for fac¬ 
simile privacy. 

Laboratory tests were made to determine the 
effectiveness of telephone systems as applied to 
facsimile. Throughout the investigation of all 
known and suggested methods of privacy for 
graphic copy, the attempt was made to con¬ 
solidate the acquired data on means of apply¬ 
ing the systems studies, and on the effectiveness 
and availability for production and other points 
of interest to the Services. 

^ Privacy Defined 

A coded facsimile subject may be considered 
to have been rendered secret when it is so coded 


105 



106 


FACSIMILE PRIVACY SYSTEMS 


that no means may be improvised to decode it, 
other than use of the applied code and equip¬ 
ment. This is the ideal condition, but one which 
was unattainable in practice. 

A coded facsimile subject may be considered 
to have been rendered private, from a military 
point of view, when at least 72 hr are required 
to reproduce the essential intelligence of the 
original subject copy. If the subject copy is a 
map, the essential intelligence is obtained when 
locations are disclosed. If the subject copy is 
type, the essential intelligence is disclosed when 
the characters become legible, regardless of de¬ 
coded subject quality. 

The relative degree of privacy of a coded 
facsimile subject may consequently be estimated 
by the number of hours required to decode its 
essential intelligence. This time may sometimes 
be reduced by subdivision of decoding opera¬ 
tions among several members of a decoding 
staff. 


Facsimile Coding Methods 

A facsimile signal is less vulnerable to unau¬ 
thorized reception than ordinary voice or code 
signals because it is unintelligible unless one 
has a recorder. With a recorder available, how¬ 
ever, which can be readily adjusted over a wide 
range of drum speeds, it would be a matter of 
less than a minute to establish the correct op¬ 
erating conditions to receive a picture. It must 
be assumed that the enemy has such apparatus 
and that, therefore, additional privacy means 
are essential for military transmission. 

Unlike voice transmission, the sending of 
intelligence by facsimile depends on two para¬ 
meters. The first is the continuous envelope of 
signals representing the shading of successive 
picture elements. The second is the information 
as to where each successive picture element 
should be printed on the recording sheet in 
order that a picture may be formed. This is 
normally supplied by moving the scanning spot 
through a fixed and simple scanning pattern 
at a known rate of speed. Obviously, privacy 
can be secured 

1. By scrambling the signal transmission, 

2. By confusing the scanning pattern. 


3. By both scrambling the signal transmis¬ 
sion and confusing the scanning pattern. 

The facsimile picture signal is ordinarily a 
modulation on a subcarrier and can be trans¬ 
mitted through the same apparatus and circuits 
as are used for voice signals. For low-speed 
facsimile the channel width required is also 
of the same order. Thus, already developed 
methods of voice scrambling can be considered 
for facsimile privacy. There are three of these, 
the first three in the following tabulation. 

Six Basic Methods 

The assigned designations below are used 
throughout the rest of the report. 

1. [A] Transposition of frequency bands. 

The spectrum of signal fre¬ 
quencies is divided into five bands 
and these are manipulated to 
secure privacy. 

2. [B] Frequency multiplication. 

All signal frequencies are mul¬ 
tiplied by a complex and change¬ 
able coding wave to produce a 
new pattern of frequencies within 
the transmitted spectrum.^^ (See 
Chapter 3.) 

3. [TDS] Time delay system. 

The signal envelope is divided 
into intervals of time, which in¬ 
tervals are variously delayed and 
then transmitted in a new order. 

When these three speech scrambling systems 
are applied to facsimile, the first two scramble 
the signal; the last one, however, is equivalent 
to a confusion of the scanning pattern and it 
can be most clearly thought of in that way. It 
is just as if the scanning spot traversed one 
part of a line of the subject, then jumped to 
another part, and so covered the whole area 
in an irregular sequence of partial scanning 
lines. 

To these already developed and self-con¬ 
tained coding means there may be added the 
following: 

4. [PR] Polarity reversal. 

Parts of the scanning line are 
reversed in polarity, from black 
to white and vice versa, according 
to a coding sequence. 




FACSIMILE PRIVACY 


107 


5. [VS] Variable speed. 

The scanning pattern is modi¬ 
fied by changing the drum speed 
by varying amounts according to 
a coding sequence. 

6. [PT] Pretransmission. 

The subject copy is itself 
scrambled by optical or other 
means before being placed on the 
scanner.^® (See Section 6.3.) 

Methods 4 and 5 involve special modifications 
of the normal facsimile equipment. Method 4 
can be thought of either as a distortion of the 
signal envelope or a confusion of the scanning 
pattern; Method 5 is obviously the latter. 
Method 6 has grown out of the activity of this 
project but is, of course, not limited to fac¬ 
simile. It would-be equally applicable to mes¬ 
sages delivered by courier. 

It should be emphasized that the very orderli¬ 
ness of the normal facsimile process makes 
difficult the successful application of privacy 
methods to picture transmission. Any cyclic 
switching operation will reveal its true time 
sequence in the facsimile reproduction. Line 
sections must be exactly fitted to avoid gaps 
or overlap. Even the white picture background 
may reveal coding changes due to slight ex¬ 
posure variations. Long, straight lines normal 
to the direction of scanning are particularly 
revealing since they serve as time references, 
and disclose periodicities. In fact, the repro¬ 
duced facsimile copy is a permanent record of 
all the optical and electrical operations which 
have been performed on the subject copy. Un¬ 
like the scrambled radio-telephone sound wave, 
it is permanently captured to lure to the fullest 
extent the ingenuity of the decipherer. 

Nomenclatuke for Various Methods 

To facilitate the designation of the various 
secrecy methods, a nomenclature was adopted 
in the final and abridged project reports in 
which the basic systems are indicated by letter 
combinations. In addition to the letters, a 
numeral is appended to differentiate between 
specific types of a basic system. If two basic 
systems are operated in combination, the letter 
group of each system will be used. These 
designations are as follows. 


Transposition of Frequency Bands [A]. The 
method of subdividing the picture signal into 
discrete frequency bands and interchanging 
these bands in the frequency spectrum in ac¬ 
cordance with a coding signal is designated by 
the letter A. A modification of this method 
includes the process of frequency inversion. 

A-1 and A-2. These designations are left open 
for experimental development. 

A-3. This is the standard commercial system 
of the Western Electric Company. 

Frequency Multiplication [B]. The method 
of transmission whereby a coding signal is 
multiplied into an intelligence signal is desig¬ 
nated by the letter B. Modifications of this basic 
system are essentially modifications of either 
the intelligence signal or the coding signal. 

B-1. Straight multiplication without altera¬ 
tion of the intelligence signal or the coding 
signal. 

B-2. An audio tone of fixed amplitude and 
of a frequency outside of the picture signal 
band has been added to the intelligence signal. 

B-3. An audio tone of fixed amplitude and 
of variable frequency outside of the picture 
signal band has been added to the intelligence 
signal. 

B-4. The normal subcarrier frequency modu¬ 
lation [SCFM] modulated with a 500-cycle 
tone. 

B-5. The normal SCFM picture signal is 
limited; i.e., the sine wave of varying frequency 
is converted to a square wave of varying fre¬ 
quency before multiplication. 

B-6. A very low frequency (less than 1 cycle 
per second) has been added to the picture 
before conversion to SCFM and subsequent 
multiplication. This results in a slow shifting 
of the picture signal band in the frequency 
spectrum. 

B-7. A method wherein the coding signal is 
changed periodically. The equipment for this 
system has been named “Myopia Mark 1.” 

Time Delay System [TD^]. Essentially, this 
method produces,a continuous transmission of 
coded signals by breaking down the normal 
signal sequence into discrete time-signal inter¬ 
vals and rearranging these intervals in a dif¬ 
ferent chronological order. Modifications of this 
system are made on the basis of the total 



108 


FACSIMILE PRIVACY SYSTEMS 


number of signal intervals that can be switched. 

TDS-1. This is the Model B magnetic-tape 
system as developed by the Bell Telephone 
Laboratories [BTL]. 

TDS-2. This is the D-Specification magnetic- 
tape system as developed by BTL. 

TDS-3. This is the C-50 magnetic-tape sys¬ 
tem as developed by BTL under Project C-50.^^ 

TDS-4. This is a simple two-head magnetic- 
tape system as used in the preliminary tests on 
this project. The time delay between the two 
heads was approximately 1/20 sec. 

Polaritij Reversals [PR]. This method is 
designated by the letters PR. Basically, it is 
a system which subdivides a continuously vary¬ 
ing picture signal into time intervals and 
determines the polarity of the transmitted 
signal throughout these intervals. The process 
can be visualized as a reversing switch con¬ 
trolled by an auxiliary or coding signal which 


PR-2. Polarity reversals in which the time 
of transmission of one polarity is less than the 
time of transmission of a scanning line. With 
this arrangement, every scanning line of the 
coded picture will resemble a chain whose alter¬ 
nate links have reversed polarity. 

PR-3. This method is similar to PR-2 except 
that the duration of the switching polarity is 
decreased until it approximates that of a pic¬ 
ture element. 

Variable Speed [ViS^] and [CF5]. In a 
variable-speed transmission system, the linear 
speed of scanning is varied in accordance with 
a coding signal. In practice, this variation may 
be in fixed steps [VS], or continuously variable 
[CVS]. It represents a modulation of the scan¬ 
ning rate, and may vary from a small per¬ 
centage of the scanning rate to 15 per cent or 
more. 

Speed deviation classifications have been 



interchanges the black and white portions of 
the picture. The degree of confusion will then 
be directly proportional to the rate of switching. 
Therefore, the applications of the basic method 
are subdivided into the following forms. 

PR-1. Polarity reversals in which the time 
of transmission of one polarity is greater than 
the time of transmission of a scanning line. 
The coded picture will then be composed of 
groups of scanning lines of alternate polarity 
and the number of scanning lines in any group 
will vary with the duration of the coding signal. 


designated as indicated below: 

VS-1. Speed change code of ± 1/2 cent. 
VS-2. Speed change code of ±1 per cent. 
VS-3. Speed change code greater than ±1 
per cent. 

VS-4. Two dimensional VS scanning for 
PT method. 

CVS-1. Continuously variable speed change 
code. 

Pretransmission [PT]. Methods wherein the 
subject copy is itself scrambled by optical or 
other means before being placed on the scanner. 
























FACSIMILE PRIVACY 


109 


Typical Samples of Coding and 
Decoding by Various 
Single Privacy Methods 

Transposition of Frequency Bands [A]. 
Figure 1 is a typical example of coding and 
decoding with the existing Western Electric 
speech privacy equipment A-3. The right por- 


Frequency Multiplication [R]. Figure 2 is a 
typical example of coding and decoding with 
the Myopia Mark I equipment. The right sec¬ 
tion shows the coded picture using 3-sec normal 
code shift; the center section, the decoded copy; 
and the left section, the normal uncoded subject 
copy. 



iAt|CfL«NA 


•«A»KiO 




4LCtCII$ 

. X 


Figure 2. Copy coded by frequency multiplication (right) and decoded (middle). 


tion of the recording is the uncoded subject 
copy; the middle section, coding by five test 
frequencies; and the left section, the decoded 
copy. 


Mathematical analysis indicates that method 
B should be subject to decoding by means of a 
filter placed at the frequency corresponding to 
either white or black, by virtue of the differen- 



Figure 3. Effect of passing coded signal of Figure 2 through narrow-band filter. Left half employed only 
one coding wave; right half used 3-sec code shifts. 


The coded section is seen to introduce a tiating action of the photocell at the boundary, 
background, without scrambling of the essen- Figure 3 shows the effect of passing the coded 
tial intelligence. The system has no privacy signal through a 100-cycle narrow-band filter 
when applied to facsimile transmission. having a 1,445-cycle mid-frequency, which cor- 












110 


FACSIMILE PRIVACY SYSTEMS 


responds to white in the recording. The left 
half of the recording utilized only one coding 
wave, the right half employed 3-sec code shifts. 
The essential intelligence of the subject copy 
is seen to be revealed by filter decoding. 


Figure 5 is the result of coding by polarity 
reversals at the rate of approximately 50 per 
scanning line. The left portion of the recording 
is the result of applying a telegraphic signal 
repeating every 80 bands, as an additional 




SPAIN 


Figure 4. Map scrambled by TDS system. Left portion is decoded version of map. 


Time Delay System [TD^]. Figure 4 is a 
typical example of coding and decoding by the 
D-Specification TDS system. The right portion 
of the picture shows a complete scramble with 
little evidence of switching periodicities. The 
left portion of the picture is the reconstructed 
subject copy resulting from the decoding opera¬ 


coding source. Boundary conditions are seen 
to convey a considerable degree of intelligence 
in both sections of the coded picture. 

Figure 6 is the decoded version of the above 
coded picture. It shows a reproduction of the 
subject copy, with good quality. 

The susceptibility of the PR method to de- 



Figure 5. Appearance of copy scrambled by polarity reversal. 


tion. D-Specification TDS is an immediately 
available system, having a high degree of view¬ 
ing privacy, and a considerable degree of de¬ 
coding privacy. 

Polarity Reversals [PR]. Figures 5 and 6 
are typical samples utilizing the PR method. 


coding by switching transients passed through 
a narrow-band filter is shown in Figure 7. 
In this recording the switch reverses polarity 
at the rate of approximately 50 times per 
scanning line. The left portion of the picture is 
normal uncoded subject copy, the middle sec- 




FACSIMILE PRIVACY 


111 


tion coded, and the right section decoded by 
passing the 1.5- to 2-kc f-m signal through 
a 100-cycle narrow-band filter having a mid¬ 
frequency of 1,785 cycles. The recording is seen 
to be completely decoded by the filter, as far 
as essential intelligence is concerned. 


drum at normal drum speed will be noted in 
the coded section of the recording. 

The CVS method, at the same maximum fre¬ 
quency deviation of ±5 per cent, is illustrated 
by Figure 9 made by Times Telephoto Equip¬ 
ment, Inc., using continuously variable drum- 



Figure 6. Decoded version of Figure 5. 


Variable Speed [FaS] and [CFS^]. A typical 
recording by the VS method is illustrated by 
Figure 8. A speed deviation having a maximum 
of ±5 per cent was applied for a fixed number 


speed shift, but with a random drum-speed 
swing. The lower section of the picture is 
normal uncoded, the middle section coded, and 
the upper section synchronously decoded. The 



Figure 7. Example showing susceptibility of PR system to decoding by passing coded signals through 
narrow-band filter. Left portion, uncoded copy; middle portion, coded by PR; right portion, copy decoded 


by use of filter. 

of drum revolutions varying from 10 to 20 
per frequency step. The lower section of the 
recording is normal uncoded, the middle section 
coded and the upper section decoded. A decoded 
strip corresponding to ten revolutions of the 


CVS method shows the highest degree of pri¬ 
vacy of the single methods. No direct electrical 
method has as yet been devised to break the 
CVS code. 

Pretransmission [PT]. The pretransmission 














112 


FACSIMILE PRIVACY SYSTEMS 


method PT has not been developed experi¬ 
mentally to the stage in ’which coded and 
decoded subject copy is available by facsimile 
transmission. The possibility of the method for 
coding purposes is shown in Eastman Kodak 
Company samples, Figure 10, in which a double 
shredding process was used. At the lower left 



Figure 8. Recording by variable speed [VS] sys¬ 
tem. Lower portion, normal uncoded; upper por¬ 
tion, decoded material. 


of Figure 11 is shown a coding by the single 
shredding process, and at the lower right a 
second shredding at right angles to the first. 

The method is limited as to its basic privacy 
by the problem of registration and skew which 
will determine the practical element size. The 
method gives promise of being particularly 
valuable as a combination method with TDS 
or CVS. 

Coding and Decoding by Tandem 
Combinations of Privacy Methods 

Polarity Reversal [PR] and Time Delay 
[TZ)5] in Combination. The PR and TDS com¬ 


bination is illustrated by Figure 12. Mixing 
of the two methods produces a high degree 
of privacy for the combination. Difficulty of 
synchronization and phasing at the scanner 
and recorder, unfortunately, renders the com¬ 
bination less attractive for practical circuit 
applications. 



Algeria 


MILES ^ 

. I 1— I » 

o XM a«e \ 


Figure 9. Scrambling by continuously varying 
drum speed. 

Transposition of Frequency Bands [A] and 
Time Delay [TD^] in Combination. The A and 
TDS combination is illustrated by Figure 13, 
made by using the standard A-3 Western Elec¬ 
tric speech privacy system with the BTL Model 
B TDS system. The right section shows normal 
recording without A or TDS; the middle sec¬ 
tion is the coded picture utilizing the coders of 
A and TDS in combination; and the left section 
is the decoded copy resulting from passing the 
coded signal through the tandem decoders. 
Reference to the coded middle section shows 
that a single system having no privacy by itself 
may be raised, by combination with another 








FACSIMILE PRIVACY 


113 


single system, far above that of either compo¬ 
nent system used singly. 

Variable Speed [F5] and Time Delay 
in Combination. The TDS and VS combination 
is illustrated by Figure 14. The single methods 
used in this combination are D-Specification 
TDS and a step-by-step variable speed VS at 
the rate of ± 1/2 per cent. A remarkable in¬ 
crease in visual scrambling is attained com¬ 
pared to using the single method alone. 

Frequency Midtiplication [B] in Combina¬ 
tion ivith Variable Speed [ViS]. Figure 15 is 
the result obtained with the B and VS combi- 


and varying the drum speeds in synchronism; 
and the left section, the uncoded subject copy. 

The B and VS combination is less attractive 
from an application viewpoint due to the rela¬ 
tive ease with which B may be decoded by 
a narrow-band filter intercepter. 

Practical Applications of 
Privacy Methods 

Transposition of frequency bands [A] is 
represented by the Western Electric Type 3-A 
speech privacy equipment. It is fixed station 
equipment of large bulk and weight. 



nation. The Myopia Mark I coder and decoder 
was used in applying the B coding. SCFM 
recording with a subcarrier frequency of 1,380 
to 1,750 cycles was applied. The coder was 
operated on a 3-sec continuous-code shift with 
constant amplitude into the unit. VS was 
applied at the rate of ± 1/2 per cent speed 
deviation. 

The right portion of this illustration is the 
coded subject; the middle section, decoded by 
passing the coded signal through the decoder 


The method depends upon fixed interchange 
of frequency bands. There is no automatic 
synchronization or phasing problem. The 
method has been shown to give no privacy 
when applied to facsimile transmission. 

Frequency multiplication [B] is represented 
by Model RCAL-l^^ weighing 32 lb and having 
a volume of approximately % cu ft. The code 
is set up on numbered dials, with provision 
for continuous change by means of a clock 
mechanism. 















114 


FACSIMILE PRIVACY SYSTEMS 



Figure 11. Pretransmission coding by single shredding (left) and by double shredding process (right). 






FACSIMILE PRIVACY 


115 


Phasing is obtained by starting the coding 
dials at a given setting and time. Synchroniza¬ 
tion between coding waves at sending and re¬ 
ceiving points is obtained from reference pulses 
transmitted over the circuit. These will require 
precise frequency standardization. 


It is probable that multipath propagation will 
destroy both the coded wave and the synchro¬ 
nizing signal. 

The unit as it stands can be used inde¬ 
pendently for voice or facsimile transmission. 


code remains fixed until the cards are changed. 

The receiving decoder is synchronized on a 
start-stop basis with the commutator of the 
sending coder by a regularly repeated pulse 
signal. The code is repeated every % sec so 
that no long time phasing is needed. The coder 


and decoder do not have to be started simul¬ 
taneously. It is computed that the precision of 
synchronization required is the same as now 
obtains on facsimile apparatus. Any skew in 
the received copy may be corrected by read- 



Figure 12. Illustration of use of TDS and polarity reversal in combination. 



Figure 13. Effect of using frequency band transposition [A] and TDS in tandem, thus increasing 
security offered by either method. 


Time delay system [TDS] is represented by 
the BTL D-Specification model weighing 25 lb, 
and having a volume of approximately % cu ft. 
Codes are set up by inserting two perforated 
cards in their appropriate boxes. A particular 


justment of the frequency standards between 
pictures. 

Distortion of the signal by multipath should 
be the same on TDS as on straight uncoded 
transmission. At the time of the project, D-Spec 


















































116 


FACSIMILE PRIVACY SYSTEMS 


TDS was available privacy equipment, an 
independent unit, equally applicable to voice 
or facsimile. 

Variable speed [VS] and continuously vari¬ 
able speed [CVS] consists of deviating the 
drum speed with respect to normal speed at a 


be exactly the same at scanner and recorder. 
This requires that the code sequence at the 
two points should be started with an estimated 
error of not more than 0.01 sec. 

Radio circuit distortions will have no more 
effect on VS and CVS than on normal facsimile. 



Figure 14. Coded and decoded copy produced by combining VS and TDS. 


coded rate, either in fixed steps or continuously 
to a maximum of ±15 per cent. The apparatus 
added to a normal facsimile station will depend 
upon the type of drum drive circuit being used. 
It may, or may not, exceed in volume the % 
cu ft already mentioned for other methods. 


They may, however, make it difficult to send 
the timing signal at the start of each message 
with the precision necessary to start the codes 
in phase. 

A special design of facsimile machine ap¬ 
pears to be the most satisfactory solution for 



Figure 15. Use of frequency multiplication (B) and variable speed (VS) in tandem. Right portion, coded 
copy; middle portion, decoded copy. 


A basic synchronization accuracy of 1 part 
in 100,000 will be required. A differential be¬ 
tween this standard and the scanner and 
recorder must be taken to provide the required 
percentages, slow or fast. The critical condition 
is that the sequence of speed variations must 


the VS and CVS method. It is consequently not 
an immediately available privacy system. 

Pretransmission method [PT] is indicated 
to be most useful as one of the basic methods 
of a tandem combination. The application 
greatly simplifies tandem transmission since 





PRETRANSMISSION FACSIMILE PRIVACY 


117 


the simultaneous use of more than one set of 
coders and decoders is not required. The result¬ 
ing privacy of TDS or VS transmission may 
thereby be greatly enhanced. 

The A-3 graphic method of Faximile, Inc., 
(Project 13.3-97)3® and the double shredding 
method suggested by the Eastman Kodak Com¬ 
pany are promising applications of the PT 
method. 

Summary of Privacy Methods 

Several kinds of coding units have been 
tested which could be built in a volume of 1 
cu ft and in mechanical form suitable for 
military service. 

Synchronizing and phasing will be critical 
problems in radio transmission. 

The B system and combinations will not 
function over radio circuits with severe multi- 
path distortion. 

Methods TDS, PR, and VS will not be seri¬ 
ously affected by multipath distortion. 

Methods A, B, and TDS can be used 
ultimately for facsimile and voice coding. 

Methods PR and VS are special to facsimile 
and would be of questionable effectiveness for 
voice coding. 

Conclusions 

In general, privacy systems which are useful 
for speech may or may not be suitable for 
facsimile. This is because the original facsimile 
signal has a regular periodicity which tends to 
reveal the code that has been applied. 

Of the three existing speech privacy systems, 
A-3 gives no privacy for facsimile transmission 
whereas TDS and Myopia Mark I are reason¬ 
ably effective. 

The privacy of several variations and com¬ 
binations of the six basic systems has been 
estimated in terms of the time required to 
decode them and will be found in the final 
report^^ together with proposed methods of 
decoding and lists of the necessary apparatus. 

Much greater privacy is obtained with a 
tandem combination of two systems than with 
either system alone. 

The best single system so far evaluated (VS) 
requires three hours for 80 per cent decoding 
of the picture. It is continuously variable drum 


speed wherein variations of not less than ±5 
per cent occur within each scanning line. 

The most effective tandem combination of 
two systems will apparently require 10 hours 
to decode 80 per cent of the picture. This is 
the combination of continuously variable drum 
speed and D-Specification TDS. Application of 
CVS with the continuously varying code TDS 
(C-50) should still further increase the time 
for decoding. This combination merits investi¬ 
gation. 

Pretransmission systems have been proposed 
during this survey, and have been evaluated. 
Such systems include coding of the subject 
copy prior to transmission by normal facsimile 
means, and decoding of the record copy after 
it has been received. In general, these methods 
can effectively code the subject copy but there 
appear to be technical difficulties in the decod¬ 
ing process. 

6 3 PRETRANSMISSION FACSIMILE 
PRIVACY 

Project 13.3-97 comprised an investigation 
of the feasibility of automatically enciphering 
and deciphering graphic material such as maps, 
photographs, drawings, etc., by strobotronic 
photography. The material is scrambled by 
transposing elementary sections of the graphic 
original before transmission. 

Basic Method Utilized 

Basically the pretransmission scrambling 
method investigated under this project is 
equivalent to cutting the copy into vertical 
strips which are transposed and remounted so 
that although the information within the strips 
is in proper order vertically, the picture is 
mutilated horizontally. After remounting the 
vertical strips, the resulting copy is cut into 
strips horizontally and after transposition of 
the elements, the copy is again assembled. Now, 
of course, the original information is hidden 
to a degree depending upon the smallness of 
the individual elements and the manner in 
which they are scrambled. Although the fea¬ 
ture was not incorporated in the model pro¬ 
duced under the instant contract, some of the 



118 


FACSIMILE PRIVACY SYSTEMS 


elements might be upside down as well as out 
of order; it is also conceivable that some of 
the elements might be reversed. 

The final product of the several transposi¬ 
tions can be sent to the receiver by courier, by 
mail, or by wire or radio facsimile or photo¬ 
graphic transmission methods. 

The Actual Mechanism 

In the machine developed and the model 
produced, the following sequence of events 
occurs. The copy to be scrambled is wrapped 
upon a cylindrical drum and rotated by a 
motor at a speed which is not critical. A single 
strip of this copy is illuminated and its image 
is projected upon a similarly mounted sheet 
of photographic paper which rotates, not con¬ 
tinuously, but in steps. 

After the first portion or ^‘frame’^ of the 
photographic paper is exposed, the drum carry¬ 
ing the sensitized paper moves one step bring¬ 
ing another frame of the paper into readiness 
for the second exposure. In the meantime the 
subject has moved to a new location, according 
to a key or code, and at the proper time this 
new portion of the original is photographed 
upon the sensitive paper. When all portions of 
the subject strip have been photographed, a 
new strip of the copy, adjacent to or otherwise 
with respect to the first strip, would be photo¬ 
graphed. In this manner all the copy would 
be impressed upon the photographic paper but 
with the elements out of order. 

After development, the negative image with 
vertical elements transposed would be used as 
an original and a new print made, this time 
with the scrambling performed in a direction 
at right angles to the first encipherment. The 
second print would be a positive like the 
original material but with all elements out of 
order. 

Increased privacy could be secured by per¬ 
forming another mutilation this time encipher¬ 
ing the transposed copy which is placed on the 
drum displaced, say, a half-strip in width. 

The deciphering is electrically and mechani¬ 
cally the inverse of the operation which 
scrambles the copy. According to the proper 
key, portions are photographed upon a sheet 
of paper which is developed to a negative image. 


As many copying operations must take place 
as occurred in scrambling the material in the 
first place and, of course, the proper key must 
be used. 

Theory of Operation 

Optically it is possible to “stop” the motion 
of a rapidly moving object at various points in 
its line of travel if it is illuminated by flashes 
of light of such short duration that for the 
period of the flash no appreciable movement 
of the object occurs. 

In the case of the graphic privacy system 
developed under this project, the moving object 
is the message to be scrambled. The flashes of 
light are furnished by the discharge of elec¬ 
trically charged capacitors through gas-filled 
strobotron tubes. Photographic means are used 
to record the results. 

The original material is scanned through 
an aperture and by its means and that of a lens 
system, i/ 2 x 2 -in. sections of the copy are photo¬ 
graphed on the sensitive paper rotated by the 
drum on which it is mounted. 

Naturally, the flashes of light must be 
properly timed in accordance with the relative 
positions of the two drums and must have the 
proper intensity. Timing is effected by elec¬ 
tronic circuits described in the final report on 
the project. 

Sixteen exposures result in transferring a 
column 2 in. wide by 8 in. high to the photo¬ 
graphic paper in coded random V 2 x 2 -in. sec¬ 
tions. Thereupon the optical assembly is moved 
axially and another 2-in. strip is photographed. 
Naturally, the consecutive sections of the 
material are not photographed in consecutive 
position on the sensitive paper, but in some 
other order determined by the key. 

The duration of the flash is about 40 psec 
so that the continuously rotated copy is, in 
effect, stationary during the photographic 
exposure. The overall effect of two cycles of 
scrambling is to produce a new positive picture 
made up of 1 / 2 -in. squares completely out of 
place with respect to each other and with 
respect to the original matter. 

Mechanical Details 

The apparatus constructed under the project 



PRETRANSMISSION FACSIMILE PRIVACY 


119 


was composed of the two drums approximately 
4 in. in diameter and 9 in. long, one being 
rotated at approximately 140 rpm by an electric 
motor through a 25 to 1 reduction gear and 
the other rotated in steps by a plunger-type 
solenoid magnet through a ratchet and pawl 
mechanism. The optical assembly was composed 
of two strobotron tubes (Sylvania R-4215) and 
a projection lens and moved along guide rails 
between and parallel to the axis of the drums. 
In the model, coding was accomplished by a 
switchboard composed of 64 flexible electric 
cords with phone tip plugs and a like number 
of tip jacks. Eastman Kodak Aero Enlarging 
Paper (mapping paper) single weight. No. 2, 
was used since this material is specially treated 
to minimize dimensional changes in processing. 

The copy drum steps once for every two 
revolutions of the subject drum. During the 
alternate revolutions, switching and other 
mechanical actions take place. In deciphering. 


the enciphered copy is placed upon the stepping 
drum, the optical assembly is reversed so that 
the same key may be used to photographically 
rearrange the elements in their original order. 

Conclusions 

Operating tests of the Model GPM-Xl ma¬ 
chine proved the validity of the principles 
involved and indicated: (1) that in a new model 
much higher drum speeds could be employed, 
(2) that it would be advisable to complete the 
encipherment in a single photographic opera¬ 
tion to avoid cumulative errors, (3) that it 
would be feasible to build a machine that would 
automatically encipher a clear message in a 
two-dimensional randomized copy consisting of 
squares at least as small as 0.2 in. on a side, 
and (4) that a newly proposed plan for chang¬ 
ing keys should be utilized. Such a new machine 
would provide a substantial term of privacy 
for graphic copy. 



Chapter 7 

MISCELLANEOUS PROJECTS 


T he final two projects considered in this 
volume are related in a general way to the 
subject matter that has gone before. Under 
Project C-52,^ Division 13 developed a crypto¬ 
graphic rotor which had certain advantages 
over the similar rotor in use at the time. 
Project C-71^ was set up to study the radio 
transmissions of German submarines in an 
endeavor to determine if frequency-modulation 
were being used in addition to the normal 
amplitude modulation. Methods were developed 
for recording and studying these signals, the 
recording apparatus being somewhat similar to 
the schemes described in the earlier portions 
of this volume. 


7 1 ROTOR FOR CRYPTOGRAPHIC USE 

Project C-52 was concerned with the design, 
development, and test of a set of rotors for 
multiple transposition coding when used on a 
suitable printer typewriter. Decoding was 
accomplished by use of a reversing switch. 

State of the Art 

At the time this project was started the 
Signal Corps had in use a typewriter type of 
coding and decoding machine which would 
automatically perform a quadruple transposi¬ 
tion ciphering each time a key was struck. 
This was accomplished by means of four disks 
(actually rotary switches), each disk having 
input and output terminals for each letter of 
the alphabet. The interconnecting wires be¬ 
tween these terminals gave the particular 
transposition for that disk. Four such disks 
were used in series. After each letter was 
coded, one or more of these disks was rotated 
to change the code for the next letter. The 
machines in use by the Signal Corps had two 
disadvantages. First, it was quite difficult to 


a Project C-52, Contract OEMsr-542, Fournier In¬ 
stitute. 

Project C-71, Contract OEMsr-880, Western Elec¬ 
tric Company, Inc. 


rotate the disks since heavy pressures had to 
be kept on the contact fingers in order that 
good electrical connections could be made. 
Second, the construction of the contacts in the 
disks was such that many troubles were en¬ 
countered with arcing and tracking across the 
insulating sectors between contact points. 
These two difficulties prevented the machines 
from being used more widely. 

Object of the Investigation 

The widespread use of codes and ciphers by 
the military forces in time of war makes it 
essential that the process of coding and decod¬ 
ing messages be made as simple as possible. 
This is usually accomplished by either mechani¬ 
cal or electrical devices or a combination of 
both. An ideal solution of this problem is a 
machine with a standard typewriter keyboard 
upon which a message may be written in plain 
language resulting in a printed version of this 
message being coded automatically by the 
machine and delivered instantaneously. The so- 
called transposition cipher is most commonly 
used, in which the letter “a” for instance, when 
struck on the keyboard, causes some other letter 
to replace it in the coded message. For purposes 
of additional security, it is arranged so that 
the next time the letter “a’" is used a different 
letter represents it than that one which was 
originally used. A wide variety of multiple 
transpositions may be used to obtain the neces¬ 
sary security. 

Such a transposition can be obtained by 
having an electrical connection made when a 
key on the keyboard is depressed. This electrical 
connection is made to a contact finger which 
touches one of twenty-six contacts on an insu¬ 
lated rotary disk. On the other side of this 
disk there are also twenty-six contacts, one for 
each letter of the alphabet. An electrical con¬ 
nection is permanently made by wires from 
contacts on one side of the disk to contacts on 
the other side in a predetermined manner to 
give the alphabetical transposition which is 
desired. Twenty-six outgoing contacts bear on 


120 



ROTOR FOR CRYPTOGRAPHIC USE 


121 


each of the rotor contacts and are connected 
to a printer circuit which causes a letter to be 
printed corresponding to the contacts through 
which the electrical circuit was made. Such an 
arrangement with one rotor disk gives a single 
alphabetical transposition. A more secure code 
can be obtained if several transposition disks 
are used in series, and are rotated in some 
predetermined fashion to make the breaking 
of the coded messages even more difficult. 


sary to use a rather strong spring on the con¬ 
tact fingers. This made it very difficult to turn 
the rotors, an operation which was necessary 
for each new letter which was transmitted. 
Since these devices were to be used in advanced 
locations where electric power was not avail¬ 
able, it was not possible always to provide some 
strong solenoid action to force this rotation 
against the necessary friction. To accomplish 
this by a straight mechanical linkage to the 



Figure 1. General view of testing equipment for cryptographic rotor, rotors in place ready for use. 


It is also possible to design an electrical 
reversing mechanism which enables the same 
machine to be used in decoding messages. The 
coded message is transcribed on the keyboard 
as it was originally received, and the plain test 
version appears automatically decoded on the 
printer tape from the machine. 

Machines of this description were in use by 
the Signal Corps at the time of the project, 
but two difficulties had been encountered. The 
first difficulty was that to make a reliable 
contact with the existing designs, it was neces- 


keyboard would have necessitated a very special 
keyboard with a long distance of travel for 
each key, and would have required the use of 
considerable force when the key was struck. 

A second difficulty was that the existing de¬ 
signs used a bakelite disk in which were molded 
brass inserts which were turned until they were 
flush with the bakelite surface. As the disks 
were rotated after each letter, the contact 
finger rubbed across the bakelite in its travel 
from one rotor contact to the adjacent one. This 
resulted in a track being made across the bake- 



122 


MISCELLANEOUS PROJECTS 


lite surface, and ultimately a low-resistance 
path was built up, which led to electrical break¬ 
down and sparking across this path. When this 
happened the rotor had to be discarded and a 
new one substituted. 

The object of this investigation was to 
develop a rotor which could be used in such a 
device without the difficulties of voltage break¬ 
down between the contacts and excessive 


design would be satisfactory. Two actual rotors 
were designed and the second which was 
smaller and less complex than the first was 
actually constructed and submitted to the 
Signal Corps. Most of the time and effort were 
spent on test equipment. After rotors and test 
equipment had been designed and constructed, 
life tests were run and demonstrations were 
made. 



Figure 2. Back view showing details of eight rotor disks. 


mechanical friction during rotation. It was 
necessary that any proposed rotor be readily 
manufacturable, and if possible, it was desired 
to make the connection between the contacts 
on each side of the rotor sufficiently flexible so 
that they could be changed readily in the field. 

Procedure 

There were two principal problems involved 
in attacking the project, (1) to design and 
construct the rotors and contact and (2) to 
devise proper test equipment to insure that the 


The coding disks and rotors constructed were 
quite easy to rotate while still giving reliable 
electrical contact as proved by the life tests. 
Substantially complete freedom from leakage 
and tracking between terminals and contacts 
was achieved. The final model should be readily 
and easily manufactured in large quantities. 
So far as is known no use was made of the 
designs or models. 

The final report^® on the project gives com¬ 
plete mechanical description including working 
drawings. 






RADIO RECORDING 


123 


7 2 RADIO RECORDING 

Project C-71 comprised an investigation of 
the possible existence of frequency modulation 
in certain telegraph transmissions and develop¬ 
ment of means for recording the transmissions. 

Summary of the Project 

The work under this project was carried out 
at the request of the Navy Department as a 
means of identifying German naval vessels by 


the characteristics of their telegraph trans¬ 
mitters. Such identification, by recordings, 
would supplement the direction-finder net set 
up to watch for enemy signals. 

Under the project more than a thousand 
transmissions were recorded by means of 
apparatus developed to detect the frequency- 
modulation in the transmissions and were 
transmitted to the Navy. 

The final report^^ of the project gives the 
means used and the results obtained. 














BIBLIOGRAPHY 

Numbers such as Div 13-302-M3 indicate that the document listed has been microfilmed and that its 
title appears in the microfilm index printed in a separate volume. For access to the index volume and to 
the microfilm, consult the Army or Navy agency listed on the reverse of the half-title page. 


1. Speech Privacy Syste'tns, Interception, Diagnosis, 

Decoding, Evaluation, OSRD 4573A, Final Report 
Project C-43, W. Koenig and C. H. G. Gray, BTL, 
Oct. 12, 1944. Div. 13-302-M3 

2. A Coding Arrangement for C-50 A-3 Privacy Sys¬ 

tem, J. M. Fraser, OSRD 4573B, Preliminary Re¬ 
port 21 of Part II, Final Report Project C-43, 
BTL, July 31, 1943. Div. 13-302-M4 

3. Methods for the Automatic Scrambling of Speech, 

G. Guanella, OSRD 4573B, Preliminary Report 5 
of Part II, Final Report Project C-43, December 
1941. Div. 13-302-M4 

4. Methods for the Automatic Scrambling of Speech, 
W. Koenig and P. W. Blye, OSRD 4573B, Pre¬ 
liminary Report 5 of Part II, Final Report 
Project C-43, BTL, December 1941. 

Div. 13-302-M4 

5. Analysis of Brown Boveri Two-Dimensional Speech 
Scrambling System, W. Koenig, OSRD 4573B, 
Preliminary Report 9 of Part II, Final Report 
Project C-43, BTL, Sept. 25, 1942. Div. 13-302-M4 

6. *‘The Carrier Nature of Speech,” Homer Dudley, 
Bell System Technical Journal, Vol. 19, October 
1940, p. 495. 

7. ''Remaking Speech,” Homer Dudley, Journal 
Acoustical Society of America, Vol. 11, October 
1939, p. 169. 

8. Speech Privacy Development, R. K. Potter, 

OSRD 201, Final Report Project C-1, BTL, June 
4, 1941. Div. 13-301.3-M2 

9. Privacy Considerations in Connection with Present 

and Future TDS Unit Designs and Recommenda¬ 
tions for New Models of TDS Privacy Equipment, 
R. K. Potter, OSRD 196, Final Report Project 
C-IA, BTL, May 20, 1941. Div. 13-301.3-Ml 

10. Continuously Coded TDS, Speech Privacy Equip¬ 
ment, Eugene B. Mechling, OSRD 2068, Final Re¬ 
port Project C-50, BTL, Nov. 1, 1943. 

Div. 13-301.31-Ml 

11. Frequency Time Division Speech Privacy System, 

L. Schott, OSRD 1725, Final Report Project C-66, 
BTL, May 29, 1943. Div. 13-301.3-M4 

12. Code Changing Attachment for TDS Speech 
Privacy Units, C. W. Carter, OSRD 1541, Final 
Report Project C-65, BTL, Apr. 30, 1943. 

Div. 13-301.3-M3 

13. Telegraphy Applied to TDS Speech Privacy Sys¬ 
tems, C. W. Carter, OSRD 1047, Final Report 
Project C-55, BTL, Oct. 3, 1942. Div. 13-304.3-Ml 

14. RCA Speech Secrecy Research, Project C-54, RCA 
Laboratories. 

Part I. Proposed Portable Speech Privacy Unit 
with High Security, A. V. Bedford, OSRD 1882, 
Feb. 2, 1943. Div. 13-301.1-Ml 


Part 11. Description of Speech Privacy Unit 
Model RCAL-1 and Related Information, A. V. 
Bedford, OSRD 3107, Oct. 30, 1943. 

Div. 13-301.1-M2 

Part III. Modification of Speech Privacy Units, 
Radio Tests and Conclusions, A. V. Bedford, 
OSRD 3395, Feb. 25, 1944. Div. 13-301.1-M3 

15. Speech Secrecy System Development, 0. M. Dun¬ 
ning, OSRD 207, Final Report Project C-15, 
Hazeltine Service Corp., Oct. 28, 1941. 

Div. 13-301.2-Ml 

16. Variable Band Shift Filter, D. F. Hoth, OSRD 
4573B, Preliminary Report 11 of Part II, Final 
Report Project C-43, BTL, Nov. 25, 1942. 

Div. 13-302-M4 

17. Large Spectrograms Made on Variable Area 
Pattern Machine, W. Koenig, OSRD 4573B, Pre¬ 
liminary Report 13 of Part II, Final Report 
Project C-43, BTL, Jan. 13, 1943. Div. 13-302-M4 

18. Method of Finding the Decoding Permutations in 

Brown Boveri Two-Dimensional Scrambling Sys¬ 
tem, W. Koenig, OSRD 4573B, Preliminary Report 
10 of Part II, Final Report Project C-43, BTL, 
Oct. 31, 1942. Div. 13-302-M4 

19. Mechanical and Numerical Aids for Cracking Re¬ 
peated Code TDS, A. D. Fowler and W. Koenig, 
OSRD 4573B, Preliminary Report 14 of Part II, 
Final Report Project C-43, BTL, Jan. 25, 1943. 

Div. 13-302-M4 

20. Experiments in Cracking Two-Dimensional 

Scrambles, W. Koenig, OSRD 4573B, Preliminary 
Report 22 of Part II, Final Report Project C-43, 
BTL, July 23, 1943. Div. 13-302-M4 

21. Evaluation of the Privacy Afforded for Non- 
Repeated Code TDS Systems (C-50 TDS), W. 

. Koenig, OSRD 4573B, Preliminary Report 26 of 
Part II, Final Report Project C-43, BTL, Nov. 
30, 1943. Div. 13-302-M4 

22. Variable Area Speech Patterns, W. Koenig, OSRD 
4573B, Preliminary Report 1 of Part II, Final 
Report Project C-43, BTL, May 21, 1942. 

Div. 13-302-M4 

23. Variable Area Pattern Machine, D. F. Hoth, OSRD 
4573B, Preliminary Report 7 of Part II, Final 
Report Project C-43, BTL, Sept. 14, 1942. 

Div. 13-302-M4 

24. Playback Machine for Variable Area Speech 

Patterns, W. Koenig, OSRD 4573B, Preliminary 
Report 12 of Part II, Final Report Project C-43, 
BTL, Jan. 6, 1943. Div. 13-302-M4 

25. Privacy Evaluation of Repeated Code TDS and 
A-3 Privacy Equipment in Tandem, W. Koenig, 




125 



126 


BIBLIOGRAPHY 


OSRD 4573B, Preliminary Report 19 of Part II, 
Final Report Project C-43, BTL, May 27, 1943. 

Div. 13-302-M4 

26. Use of Monotone or Whispered Speech on Privacy 

Systems, W. Koenig, OSRD 4573B, Preliminary 
Report 16 of Part II, Final Report Project C-43, 
BTL, Feb. 25, 1943. Div. 13-302-M4 

27. Evaluation of the Security Afforded by the RCA- 
Bedford Speech Privacy System, W. Koenig, OSRD 
4573B, Preliminary Report 18 of Part II, Final 
Report Project C-43, BTL, May 26, 1943. 

Div. 13-302-M4 

28. Decoding Equipment for TDS, Split Band, and 

Two-Dimensional Speech Scrambling System, W. 
Koenig, OSRD 4573B, Preliminary Report 15 of 
Part II, Final Report Project C-43, BTL, Jan. 25, 
1943. Div. 13-302-M4 

29. Playback Devices for Spectrograms, L. Y. Lacy, 
OSRD 4573B, Preliminary Report 17 of Part II, 
Final Report Project C-43, BTL, Mar. 30, 1943. 

Div. 13-302-M4 

30. Analysis of Generalized TDS Codes, A. D. Fowler, 
OSRD 4573B, Preliminary Report 3 of Part II, 
Final Report Project C-43, BTL, June 16, 1942. 

Div. 13-302-M4 

31. Analysis of Self-Converse TDS Codes, A. D. 

Fowler, OSRD 4573B, Preliminary Report 6 of 
Part II, Final Report Project C-43, BTL, Aug. 
19, 1942. Div. 13-302-M4 

82. TDS Field Decoding Test at Camp Coles, W. 
Koenig and A. E. Ruppel, OSRD 4573B, Pre¬ 
liminary Report 24 of Part II, Final Report 
Project C-43, BTL, Oct. 19, 1943. Div. 13-302-M4 
33. Part II, Final Report on Project C-43, containing 
all the preliminary reports, most of which are in¬ 
cluded in this bibliography. Those reports not so 
included are: 

No. 2. Results of Experimental Intercept Work 
at Holmdel, New Jersey and Point Reyes, Cali¬ 
fornia, P. W. Blye and L. Y. Lacy, OSRD 4573B, 
BTL, May 29, 1942. Div. 13-302-M4 

No. 4 Sonovox Secrecy System, L. Y. Lacy, OSRD 
4573B, BTL, July 2, 1942. Div. 13-302-M4 

No. 8. Speech Scrambling System Proposed by 
Frank D. Lewis, W. Koenig and P. W. Blye, OSRD 
4573B, BTL, Aug. 29, 1942. Div. 13-302-M4 

No. 20. Speech Privacy and Synchronizing Sys¬ 
tem Devised by Captain Henry P. Hutchinson, 
D. O. Slater, OSRD 4573B, BTL, July 31, 1943. 

Div. 13-302-M4 

No. 23. Intercept Work at Point Reyes, California, 
H. Kahl, OSRD 4573B, BTL, July 26, 1944. 

Div. 13-302-M4 

No. 25. Interception of Enemy Radiotelephone 
Communications Employing Privacy Systems; 
Problems, Procedures and Tools, W. Koenig, OSRD 
4573B, BTL, Oct. 12, 1943. Div. 13-302-M4 


34. Speech Privacy Problems, A. D. Fowler, OSRD 

5686, Final Report Project 13-106, BTL, Aug. 18, 
1945. Div. 13-300-Ml 

35. Spectrographs for Field Decoding Work, C. H. G. 

Gray, OSRD 3824, Final Report on Project 13.3-86, 
BTL, May 31, 1944. Div. 13-302.1-M3 

36. Facsimile Privacy, H. H. Beverage, OSRD 1881, 

Final Report Project C-73, RCA Laboratories, 
June 7, 1943. Div. 13-303-Ml 

Abridged Report, OSRD 2005, Oct. 15, 1943. 

Div. 13-303-M2 

37. Investigation of Pre-Transmission Facsimile 

Privacy Methods, J. V. L. Hogan, OSRD 6346, 
Final Report Project 13.3-97, Faximile, Inc., June 
30, 1945. Div. 13-303-M3 

38. Development of a Rotor for Cryptographic Use, 
Harner Selvidge, OSRD 1607, Final Report Project 
C-52, Fournier Institute, May 1, 1943. 

Div. 13-304.1-Ml 

39. Radio Recording, A. M. Curtis, Final Report 
Project C-71, BTL, June 15, 1943. Div. 13-304.2-Ml 

40. Control Circuits for the Sound Spectrograph, W. 
Koenig, OSRD 4573B, Preliminary Report 27 of 
Part II, Final Report Project C-43, BTL, Apr. 

29, 1944. Div. 13-302-M4 

41. The Sound Spectrograph, A Time-Frequency- 
Intensity Analyzer, W. Koenig, Preliminary Re¬ 
port Project C-43, BTL, Oct. 1, 1943. 

Div. 13-302.1-M2 

42. Operating Notes for Spectrograph Model Nos. 2 
and 3, R. G. McCurdy, Project C-43, BTL, Jan. 

30, 1943. Div. 13-302.1-Ml 

43. Notes on Description and Operation of Special 
Speech Privacy Decoding Equipment Used at Point 
Reyes, California, 0. M. Akey and H. Kahl, 
Project C-43, BTL, June 30, 1944. Div. 13-302-M2 

44. Speech Privacy Decoding, OSRD 386, Final Re¬ 
port Project C-32, BTL, Jan. 31, 1942. 

Div. 13-302-Ml 

45. Interception of Enemy Radiotelephone Communi¬ 
cations Employing Privacy Systems; Problems, 
Procedures and Tools, W. Koenig, OSRD 4573B, 
Preliminary Report 25 of Part II, Final Report 
Project C-43, BTL, Oct. 12, 1943. Div. 13-302-M4 

46. Results of Experimental Intercept Work at 

Holmdel, New Jersey and Point Reyes, California, 
P. W. Blye and L. Y. Lacy, OSRD 4573B, Pre¬ 
liminary Report 2 of Part II, Final Report Project 
C-43, May 29, 1942. Div. 13-302-M4 

47. Intercept Work at Point Reyes, California, H. 

Kahl, OSRD 4573B, Preliminary Report 23 of 
Part II, Final Report Project C-43, BTL, July 26, 
1944. Div. 13-302-M4 

48. Memo of Changes in Speech Privacy Units as Re¬ 
leased to the Army by RCA Laboratories, A. V. 
Bedford, Project C-54, RCA, Oct. 19, 1944. 

Div. 13-301.1-M4 



OSRD APPOINTEES 


DIVISION 13 
Chief 

C. B. JOLLIFFE (December 1942 to May 1945) 
Haraden Pratt (May 1945 to May 1946) 


Deputy Chief 
K. C. Black 


J. L. Allison 
C. F. Dalziel 


K. C. Black 
0. E. Buckley 
J. H. Dellinger 
W. L. Everitt 
G. C. Pick 
R. H. George 
C. H. G. Gray 
A. Hazeltine 


Technical Aides 


Members 


J. F. McClean 
A. F. Murray 


J. A. Hutchinson 

C. M. Jansky 
L. F. Jones 

D. G. Little 
R. K. Potter 
H. Pratt 

C. A. Priest 
F. M. Ryan 


Section Heads 


Section 13.1 

Direction Finding 

L. F. Jones 

Section 13.2 

Radio Propagation Problems 

J. H. Dellinger 

Section 13.3 

Speech Secrecy 

R. K. Potter 

Section 13.4 

Special Communications Problems 

C. A. Priest 

Section 13.5 

Precipitation Static 

H. Pratt 

Section 13.6 

Miscellaneous Projects 

D. G. Little 


L. V. Berkner 

Consultants 

E. D. Blodgett 

H. H. Beverage 


D. G. Little 


R. K. Potter 



Interference Reduction Committee 


K. C. Black 
H. D. Doolittle 
R. G. Fluharty 
A. Hazeltine 


J. C. R. Licklider 
C. T. Morgan 
A. F. Murray 
S. S. Stevens 


0. W. Towner 


CONTRACT NUMBERS, CONTRACTORS, AND SUBJECT OF CONTRACTS FOR DIVISION 13 


Contract 

Number 

Name and Address of Contractor 

Subject 

Refer to 
Chapters 

NDCrc-125 

Western Electric Company, Inc. 
New York, N. Y. 

Speech secrecy system. 

2 

NDCrc-196 

Western Electric Company, Inc. 
New York, N. Y. 

Continuation of speech secrecy system. 

2 

NDCrc-139 

Hazeltine Service Corporation 
New York, N. Y. 

Hazeltine speech secrecy system. 

3 

OEMsr-230 

Western Electric Company, Inc. 
New York, N. Y. 

Decoding speech codes. 

4 

OEMsr-435 

Western Electric Company, Inc. 
New York, N. Y. 

Continuation of decoding speech codes. 

1, 3,4 

OEMsr-490 

Western Electric Company, Inc. 
New York, N. Y. 

Improved speech secrecy. 

2,6 

OEMsr-542 

Fournier Institute 

Lemont, Ill. 

Development of rotor for cryptographic use. 

6 

OEMsr-592 

Radio Corporation of America 
Camden, N. J. 

Speech secrecy research. 

3, 5,6 

OEMsr-628 

Western Electric Company, Inc. 
New York, N. Y. 

Telegraphy applied to TDS speech secrecy system. 

2 

OEMsr-782 

Western Electric Company, Inc. 
New York, N. Y. 

Code changing attachment for TDS speech privacy 
unit. 

2 

OEMsr-795 

Western Electric Company, Inc. 
New York, N. Y. 

Frequency-time division speech analyzer. 

2 

OEMsr-880 

Western Electric Company, Inc. 
New York, N. Y. 

Radio recording. 

6 

OEMsr-837 

Radio Corporation of America 
Camden, N. J. 

Facsimile secrecy. 

6 

OEMsr-1110 

Western Electric Company, Inc. 
New York, N. Y. 

Spectrographs for field decoding work. 

5 

OEMsr-1202 

Faximile, Inc. 

New York, N. Y. 

Graphic privacy system. 

6 

OEMsr-1440 

Western Electric Company, Inc. 
New York, N. Y. 

Speech privacy problems. 

5 


128 


SECREn 








SERVICE PROJECT NUMBERS 

The projects listed below were transmitted to the Executive 
Secretary, NDRC, from the War or Navy Department through 
either the War Department Liaison Officer for NDRC or the 
Office of Research and Inventions (formerly the Coordinator of 
Research and Development), Navy Department. / 


Service 

Project 

Number 

/ 

Subject 

Refer to 
Chapters 

SC-12 

Speech secrecy system. 

2 

SC-12 

Continuation of speech secrecy system. 

2 

SC-12 

Hazeltine speech secrecy system. 

3 

SC-12 

Improved speech secrecy. 

2, 6 

SC-12 

Speech secrecy research. 

3, 5, 6 

SC-12 

Code changing attachment for TDS speech secrecy system. 

2 

SC-12 

Frequency-time division speech analyzer. 

2 

SC-19 

Telegraphy applied to TDS speech secrecy system. 

2 

SC-25 

Development of rotor for cryptographic use. 

6 

SC-28 

Decoding speech codes. 

4 

SC-28 

Continuation of decoding speech codes. 

1, 3, 4 

SC-43 

Facsimile secrecy. 

6 

NS-130 

Radio recording. 

6 

NS-134 

Graphic privacy system. 

6 

NS-349 

Speech privacy problems. 

5 


129 














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INDEX 


The subject indexes of all STR volumes are combined in a master index printed in a separate volume. For 
access to the index volume consult the Army or Navy Agency listed on the reverse of the half-title page. 


A2 speech scrambling system, 41 
A3 facsimile privacy system, 107 
A3 speech scrambling system, 21 
A4 phase reversal system, speech 
scrambling 

cryptographic decoding, 57 
noncryptographic decoding, 42 
spectrograms, 88 

A5 split-phase system, speech 
scrambling, 41, 89 
“Aided tracking” for interception 
of speech privacy systems, 40 
Amplitude representation for 
spectrographic decoding of 
speech, 64-65 

Articulation tests for TDS speech 
scrambling systems, 21 
Audio frequency code-waves, RCA- 
Bedford speech privacy sys¬ 
tem, 25 

Automatic code changing, 17-18 
Automatic decoding of speech, 44-48 
frequency band methods, 44 
multiplication systems, 46 
parallel-automatic method, 46 
summary, 47 
wobble band method, 45 
Automatic trial systems for speech 
decoding, 83 

B1 facsimile privacy system, 107 
B2 band displacement system, 
speech scrambling, 44 
B2 facsimile privacy system, 107 
B3 facsimile privacy system, 107 
B3 wobble band displacement sys¬ 
tem, speech scrambling, 40 
B4 double modulation system, 
speech scrambling, 57 
B4 facsimile privacy system, 107 
B5 facsimile privacy system, 107 
B6 facsimile privacy system, 107 
B7 facsimile privacy system, 107 
Band displacement speech scram¬ 
bling systems 
B2; 44 
decoding, 42 
Hazeltine, 33-34 
spectrograms, 89 
superposition, 43 

Band splitting speech scrambling 
systems 
Dl; 41, 57 


D2; 43, 57 
description, 3-4, 73 
Bedford type speech scrambling 
systems 

see RCA-Bedford speech privacy 
system 

Bell Telephone Laboratories 

D-specification model, facsimile 
privacy system, 115 
sound spectrograph, 35 
TDS facsimile privacy systems, 
107-108 

British modulator type 2C speech 
scrambling system, 100-101 
British two-dimensional speech pri¬ 
vacy system, 101, 103 

Cl re-entrant inversion system, 
speech scrambling, 48 
C2 continuously varied re-entrant 
displacement system, speech 
scrambling, 89 

C2 triple modulation system, speech 
scrambling, 57 

Channel mixing systems, speech 
scrambling 
LI, L2, L3; 59, 89 
multiple transmission paths, 11- 
12 

noncryptographic decoding, 43 
Code waves, RCA-Bedford speech 
privacy system, 28-31 
Code-changing attachment for 
portable TDS speech scram¬ 
bling, 22-23 

Code-changing method, RCA-Bed¬ 
ford speech privacy system, 
29 

Coding systems 
see Speech privacy systems 
Compandor for RCA-Bedford 
speech privacy system, 27, 29 
Continuously coded TDS systems, 
speech scrambling, 17-20 
automatic code changing, 17-18 
equipment, 19 

frequency-band shift system, 20 
number of sequences, 20 
ten-element system, 18 
Continuously varied re-entrant dis¬ 
placement system, speech 
scrambling, 88 

“Cracking” speech scrambling sys¬ 
tems 


frequency-TDS, 21 
portable TDS, 14 
RCA-Bedford, 32 
time measurements, 84 

Cryptographic decoding of speech, 
48-61, 120-122 
applications, 57-59 
message, 59 

oscillographic traces, 54 
playback, 59-61 
rectification, 56-57 
“rotor,” 120-122 
spectrograms, 48-51 
summary, 48 

variable-area patterns, 52-54 
visual, 54-56 

CVSl (continuously-variable), fac¬ 
simile privacy system, 108, 
112 

Cyclic speech coding system, 85 

Dl band splitting system, speech 
scrambling, 41, 57 

D2 band splitting system, speech 
scrambling, 43, 57 

Decoding methods, speech communi¬ 
cation, 35-99, 104 
automatic methods, 44-48, 83 
cryptographic, 48-61 
equipment, 104 
evaluation, 84-86 
history, 35-36 

instantaneous speech patterns, 78 
interception, 36-40 
noncryptographic, 40-48 
partial matching system, 76-78 
sound spectrograph, 61-83 
unknown signals, 86-89 

Delay networks for speech privacy 
system, 28-31 

Directive antennas for intercepted 
signals, 37 

Double modulation speech scram¬ 
bling system 

automatic decoding, 45-46 
B4; 57 

spectrogram, 88 

Double-sideband radio transmis¬ 
sion, 38 

Eastman Kodak Company, pre¬ 
transmission facsimile pri¬ 
vacy system, 112, 117 


131 


132 


INDEX 


Enemy privacy systems, captured 
sets, 40 

Equalizer unit for RCA-Bedford 
speech privacy system, 29 

FI speech scrambling system, de¬ 
layed subbands, 41 
F2 TDS system, repeated code, 
speech scrambling, 57 
F3 TDS system, nonrepeated code, 
speech scrambling, 57, 89 
F4 speed variations system, speech 
scrambling, 45, 58 
Facsimile privacy systems, 105-119 
see also under name of system 
background, 105-106 
CVS-1 system, 108, 111 
frequency band transposition 
method, 106, 107, 109, 113 
methods, 106-117 
polarity reversal systems, 106, 
108, 110-111 

pretransmission, 107, 108, 111- 

112, 116-119 
RCAL-1 system, 113 

TDS system, 106-108, 110, 115 
variable speed system, 107, 108, 

113, 116 

Western Electric system (A3), 
107 

Faximile, Inc., facsimile privacy 
system, 117 

Filters for spectrographs, 66-69 
Fixed-code speech scrambling sys¬ 
tems, 100-101 

Foreign language records for 
speech decoding, 83 
Frequency multiplication in fac¬ 
simile privacy system 
applications, 113 
description, 106 
Myopia Mark I; 109-110 
nomenclature, 107-108 
Frequency substitution systems, 
speech scrambling 
band-splitting, 3-4, 33-34, 73 
double modulation, 2 
single modulation, 1-2 
time division multiplex (TDM), 
4-5 

triple modulation, 2-3 
Frequency-band shift speech scram¬ 
bling systems, 7, 20, 44 
Frequency-band transposition in 
facsimile privacy systems 
Al, A2, A3; 107 
applications, 113 
description, 106 
samples, 109 


Frequency-time division systems, 
speech scrambling, 21-22 

G1 tape plus modulation system, 
speech scrambling, 58, 89 
G2 tape plus modulation system, 
speech scrambling, 58 
G3 tape plus modulation system, 
speech scrambling, 58 
G4 tape plus modulation system, 
speech scrambling, 58 
G5 tape plus modulation system, 
speech scrambling, 89 
G6 tape plus modulation system, 
speech scrambling, 89 
Gabrilovitch, L. E., speech privacy 
systems, 102 

German vessel detection by tele¬ 
graph transmitter identifica¬ 
tion, 123 

GPM-XI, pretransmission facsimile 
privacy system, 119 
Graphic copy, scrambling systems 
see Facsimile privacy systems 
Graphic patterns in spectrographic 
speech decoding, 69-71 

HI wave form distortion system, 
speech scrambling 
automatic decoding, 46 
cryptographic decoding, 58-59 
spectrograms, 88 

H2 wave form distortion system, 
speech scrambling, 44, 59 
H3 wave form distortion system, 
speech scrambling, 44, 59 
Hazeltine band displacement sys¬ 
tem, speech scrambling, 33- 
34 

High-power signal interception, 36 
High-security speech privacy sys¬ 
tems, 32 

Inflection effects in diagnosing pri¬ 
vacy systems, 63 

Interception of speech privacy sys¬ 
tems, 36-40 

decoding devices, 39-40 
receiver sets, 37-38 
recording methods, 39 
signal quality, 36-37 
types of radio systems, 36, 38 
“Interlace,” TDS speech scrambling 
system, 6-7 

Inversion systems, single modula¬ 
tion speech scrambling, 1, 
71, 73 

J1 masking system, speech scram¬ 
bling, 59 


J2 masking system, speech scram¬ 
bling, 44 

J3 tone sequence system, speech 
scrambling, 41 

K1 vocoder system, speech scram¬ 
bling, 59, 89 

K2 vocoder system, speech scram¬ 
bling, 59, 89 

K3 vocoder system, speech scram¬ 
bling, 59, 89 

K4 vocoder system, speech scram¬ 
bling, 59, 89 

LI channel mixing system, speech 
scrambling, 43, 59 
L2 channel mixing system, speech 
scrambling, 43, 59 
L3 channel mixing system, speech 
scrambling, 59, 89 
Level compression in spectrographic 
decoding of speech, 63-64 
Level modulation systems, speech 
scrambling, 42, 44 
Limiter, speech decoding, 42 
Linguaphone, speech decoding, 83 
Low-power signal interception, 36 

Magnetic tape speech recording 
systems 

see Time division speech scram¬ 
bling (TDS) system 
Masking systems, speech scram¬ 
bling 
Jl; 59 
J2; 44 

summary, 9-10, 103 
Military strategy, decoding speech, 
85-86 

Model RCAL-1, privacy unit, 29-31, 
113 

Modulation systems, speech scram¬ 
bling, 1-3 
double, 2 
single, 1-2 
triple, 2-3 

Modulator Type 2C speech record¬ 
ing, 100-101 

Multiplication system (HI) speech 
scrambling 

automatic decoding, 46 
cryptograph decoding, 58-59 
spectrograms, 88 

Multivibrator for RCA-Bedford 
speech privacy system, 28 
Musical effects in spectrographic 
speech decoding, 68 
“Myopia Mark I” for facsimile 
privacy system, 107, 109 



INDEX 


133 


Networks for code-waves of RCA- 
Bedford speech privacy sys¬ 
tem, 28, 31 

New Zealand switched-band speech 
privacy system, 101-102 
Noncryptographic decoding of 
speech, 40-48, 83 
automatic, 44-48, 83 
captured set, 40 
compromise methods, 41-44 
Nonrepeated code systems, speech 
scrambling, 57, 84-85, 89 
Nonscrambled speech, Hazeltine 
band displacement system, 
34 

Oscillograms for cryptographic de¬ 
coding of speech, 54 

Parallel-automatic speech decoding 
method, 46 

Peak chopper for speech decoding, 
42 

Phase varied inverter-distorter 
speech secrecy set, 102 
Phase-reversal speech scrambling 
system 

cryptographic decoding, 57 
noncryptographic decoding, 43 
spectrograms, 88 

Phonographic recordings for speech 
scrambling systems, 103-104 
Playback, cryptographic decoding 
of speech, 59-61 

Polarity reversal (PR) system, 
facsimile privacy 
description, 106, 108 
samples, 110-111 

Portable TDS systems speech 
scrambling, 13-17, 22-23 
PR-1 facsimile privacy system, 108 
PR-2 facsimile privacy system, 108 
PR-3 facsimile privacy system, 108 
Pretransmission facsimile privacy 
system, 107, 108, 111-112 
equipment, 118-119 
evaluation, 119 
method, 117-118 
operation, 118-119 
Privacy systems 

see Facsimile privacy systems; 
Speech privacy systems 
“Private” coded facsimile subjects, 
105 

Radio recordings of telegraph 
transmissions, 123 
Radio transmission, interception 
see Interception of speech pri¬ 
vacy systems 


RCA-Bedford speech privacy sys¬ 
tem, 25-33, 103 

code waves generated by delay 
network, 28, 29-31 
code-changing method, 29 
compandor, 27 
decoding, 32, 103 
present status, 32-33 
principles, 25 

synchronization of delay net¬ 
work, 29 

wave multiplier, 25 

RCAL-1 facsimile privacy system, 
113 

Receiving sets for code intercep¬ 
tion, 37-38 

Recording methods, speech scram¬ 
bling systems, 39, 103 

Rectification methods for speech de¬ 
coding, 42, 56-57 

Re-entrant inversion system, speech 
scrambling, 48 

Repeated code system, speech 
scrambling, 57 

Repeated code waves of RCA-Bed¬ 
ford speech privacy system, 
29-31 

Rotating commutator for “inter¬ 
lace” speech scrambling sys¬ 
tem, 6 

Rotors for cryptography, 120-122 

Scanning filters for cryptographic 
decoding of speech, 51-52, 
66-69 

Scrambling systems for speech 
privacy 

see Speech privacy systems 

Secrecy systems 

see Facsimile privacy systems; 
Speech privacy systems 

“Secret” coded facsimile subjects, 
105 

Self-decoding time division scram¬ 
bling system, 6 

Signal quality, interception meth¬ 
ods, 36-37 

Single modulation speech scram¬ 
bling systems, 1-2 

Sound spectrograms, 87-99 
alternate inversion, 91 
backwards, 98 
channel-mixing, 99 
fixed displacement, 91 
modulation sidebands, 90 
multiplication, 98 
re-entrant inversion, 92 
scrambled speech, 88 
simple inversion, 90 


speed wobble, 97 
split band scramble, 93, 94 
subbands delayed, 95 
TDS combined with split-band 
scramble, 96 

time and frequency measure¬ 
ments, 88 

time division multiplex, 94, 95 
two dimensional scramble, 97 
wobbled displacement, 92 
Sound spectrograph, 61-83 
amplitude, 64-65 
applications, 66-83 
description, 65-66 
diagnosis, 88-89 
history, 61 
improvements, 64 
level compression, 63-64 
measurements, 87 
operation, 62-63 

Speech patterns for scrambling 
systems, 71-74, 78 
Speech privacy systems 

see also under name of system 
band displacements, 40, 42-44, 89 
band-splitting, 3-4, 33-34, 73 
British systems, 100-101, 103 
channel-mixing, 11-12, 43, 59, 89 
continuously coded, 17-20 
diagnosis of unknown systems, 
86-99 

double modulation, 2, 45-46, 57, 
89 

evaluation, 84-86 
Hazeltine band displacement, 33- 
34 

interception, 36-40 
inversion systems, 1, 48, 71, 73 
masking, 9-10, 44, 59, 103 
modulation systems, 1-3 
multiplication systems, 42, 46, 88 
phase reversal, 42, 57, 88 
portable systems, 13-17, 22-23 
RCA-Bedford, 25-33, 103 
recording methods, 39, 103 
repeated code, 57 
single modulation, 1-2 
speed variations, 45, 58 
split phase, 41, 89 
summary list, 47 
switched band, 101-102 
tape recording, 5-6, 13-24, 52-56, 
74-78 

time division multiplex (TDM), 

4- 5 

time division scrambling (TDS), 

5- 6, 13-24, 52-56, 74-78 
tone sequence, 41 

triple modulation, 2-3 
two-dimensional, 101, 103 



134 


INDEX 


unrepeated code, 57, 84-85, 89 
vocoder, 10-11, 45, 54, 59, 89 
wave for modification, 8, 58 
Speed variations speech scrambling 
system, 45, 58 

Spill-over elfects in cryptographic 
decoding of speech, 51, 64 
Split-band systems, speech scram¬ 
bling 

Dl; 41, 57 
D2; 43, 57 
description, 3-4, 73 
Split-phase speech scrambling sys¬ 
tem, 42, 88 

Spread-band radio transmission, 38 
Stepped displacement speech scram¬ 
bling systems 
B2; 45 
decoding^ 42 
Hazeltine, 33-34 
spectrograms, 89 
superposition, 43 

Subcarrier frequency modulation 
systems, facsimile privacy, 
108 

Superposition for speech decoding, 
43-44 

“Suppressed carrier” radio trans¬ 
mission, 38 

Switched band speech privacy sys¬ 
tem, 101-102 

Synchronizing blanking circuits for 
RCA-Bedford speech privacy 
system, 29 

Synthetic speech in Vocoder sys¬ 
tems, 10-11 

Tandem transmission in facsimile 
privacy systems, 116, 117 
Tape plus modulation systems, 
speech scrambling, 58, 89 
Tape recording systems 

see Time division speech scram¬ 
bling system (TDS) 

TDM (time division multiplex) 
speech privacy system, 4-5 


DECLaSSTTiTBn 
Bz authority Secretary 


SEP 2 31960 


TDS facsimile privacy systems 
see Time delay system (TDS), 
facsimile privacy 

TDS speech scrambling system 
see Time division speech scram¬ 
bling (TDS) system 

Telegraphy transmission in speech 
privacy systems, 23-24, 39 

Ten-element TDS speech scram¬ 
bling system, 18 

Time delay system (TDS), fac¬ 
simile privacy 
applications, 113-116 
description, 106 
nomenclature, 107-108 
samples, 110 

Time division multiplex (TDM) 
system, speech scrambling, 
4-5 

Time division speech scrambling 
(TDS) systems 

code-changing attachment for 
portable TDS, 22-23 
continuously coded TDS, 17-20 
decoding, 74-78 
“interlace,” 6-7 

matching variable — area pat¬ 
terns, 52-54 

portable TDS, 13-17, 22-23 
summary, 5-6, 13 
telegraphy, 23-24 
visual methods, 54-56 

Times Telephote Equipment, Inc., 
facsimile privacy methods, 
111 

Tone sequence system (J3) speech 
scrambling, 41 

Transposition coding of speech. 


Two-dimensional speech privacy re¬ 
cording system, 7, 100-101, 
103 

Unknown systems, diagnostic 
methods, 86-89 
illustrations, 88-89 
introduction, 86-87 
spectrograms, 87 

Unscrambling methods, speech sys¬ 
tems 

see Decoding methods, speech 
communication 

Variable speed (VS) system, fac¬ 
simile privacy 
applications, 116-117 
description, 107 
nomenclature, 108 
samples, 110-112 

Variable-area patterns for crypto¬ 
graphic decoding of speech, 
52-54, 78 

Visual methods, cryptographic de¬ 
coding of speech, 54-56, 61 

Vocoder speech scrambling sys¬ 
tems, 10-11 

automatic decoding, 45 
description, 10-11 
Kl, K2, K3 and K4 systems, 59 
oscillographic traces, 54 
spectrograms, 89 

Voice scrambling systems 
see Speech privacy systems 

VS facsimile privacy systems 

see Variable speed (VS) sys¬ 
tem, facsimile privacy 


120-122 

Transposition of frequency bands 
in facsimile privacy sys¬ 
tems, 107, 109 

Triple modulation speech scram¬ 
bling system, 2-3 

“Twin-channel” radio transmission, 
38 

Two-channel speech privacy system, 
2 


Wave form modification systems, 
speech scrambling, 8, 58 
Wave form traces for decoding 
TDS, 76 

Wave multiplier for RCA-Bedford 
speech privacy system, 25-26 
Western Electric Company, A-3 
facsimile privacy system, 107 
Wobble band displacement system. 


Of 



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